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THE ANCIENT
LIFE-HISTORY
OF THE EARTH

Chapter 11:

THE THE DEVONIAN AND OLD RED SANDSTONE PERIOD.

Between the summit of the Ludlow formation and the strata which are universally admitted to belong to the Carboniferous series Page 133 is a great system of deposits, to which the name of "Old Red Sandstone" was originally applied, to distinguish them from certain arenaceous strata which lie above the coal ("New Red Sandstone"). The Old Red Sandstone, properly so called, was originally described and investigated as occurring in Scotland and in South Wales and its borders; and similar strata occur in the south of Ireland. Subsequently it was discovered that sediments of a different mineral nature, and containing different organic remains, intervened between the Silurian and the Carboniferous rocks on the continent of Europe, and strata with similar palæontological characters to these were found occupying a considerable area in Devonshire. The name of "Devonian" was applied to these deposits; and this title, by common usage, has come to be regarded as synonymous with the name of "Old Red Sandstone." Lastly, a magnificent series of deposits, containing marine fossils, and undoubtedly equivalent to the true "Devonian" of Devonshire, Rhenish Prussia, Belgium, and France, is found to intervene in North America between the summit of the Silurian and the base of the Carboniferous rocks.

Much difficulty has been felt in correlating the true "Devonian Rocks" with the typical "Old Red Sandstone"—this difficulty arising from the fact that though both formations are fossiliferous, the peculiar fossils of each have only been rarely and partially found associated together. The characteristic crustaceans and many of the characteristic fishes of the Old Red are wanting in the Devonian; whilst the corals and marine shells of the latter do not occur in the former. It is impossible here to enter into any discussion as to the merits of the controversy to which this difficulty has given origin. No one, however, can doubt the importance and reality of the Devonian series as an independent system of rocks to be intercalated in point of time between the Silurian and the Carboniferous. The want of agreement, both lithologically and palæontologically, between the Devonian and the Old Red, can be explained by supposing that these two formations, though wholly or in great part contemporaneous, and therefore strict equivalents, represent deposits in two different geographical areas, laid down under different conditions. On this view, the typical Devonian rocks of Europe, Britain, and North America are the deep-sea deposits of the Devonian period, or, at any rate, are genuine marine sediments formed far from land. On the other hand, the "Old Red Sandstone" of Britain and the corresponding "Gaspé Group" of Eastern Page 134 Canada represent the shallow-water shore-deposits of the same period. In fact, the former of these last-mentioned deposits contains no fossils which can be asserted positively to be marine (unless the Eurypterids be considered so); and it is even conceivable that it represents the sediments of an inland sea. Accepting this explanation in the meanwhile, we may very briefly consider the general succession of the deposits of this period in Scotland, in Devonshire, and in North America.

In Scotland the "Old Red" forms a great series of arenaceous and conglomeratic strata, attaining a thickness of many thousands of feet, and divisible into three groups. Of these, the Lower Old Red Sandstone reposes with perfect conformity upon the highest beds of the Upper Silurian, the two formations being almost inseparably united by an intermediate series of "passage-beds." In mineral nature this group consists principally of massive conglomerates, sandstones, shales, and concretionary limestones; and its fossils consist chiefly of large crustaceans belonging to the family of the Eurypterids, fishes, and plants. The Middle Old Red Sandstone consists of flagstones, bituminous shales, and conglomerates, sometimes with irregular calcareous bands; and its fossils are principally fishes and plants. It may be wholly wanting, when the Upper Old Red seems to repose unconformably upon the lower division of the series. The Upper Old Red Sandstone consists of conglomerates and grits, along with a great series of red and yellow sandstones—the fossils, as before, being fishes and remains of plants. The Upper Old Red graduates upwards conformably into the Carboniferous series.

The Devonian rocks of Devonshire are likewise divisible into a lower, middle, and upper division. The Lower Devonian or Lynton Group consists of red and purple sandstones, with marine fossils, corresponding to the "Spirifer Sandstein" of Germany, and to the arenaceous deposits (Schoharie and Cauda-Galli Grits) at the base of the American Devonian. The Middle Devonian or Ilfracombe Group consists of sandstones and flags, with calcareous slates and crystalline limestones, containing many corals. It corresponds with the great "Eifel Limestone" of the Continent, and, in a general way, with the Corniferous Limestone and Hamilton group of North America. The Upper Devonian or Pilton Group, lastly, consists of sandstones and calcareous shales which correspond with the "Clymenia Limestone" and "Cypridina Shales" of the Continent, and with the Chemung and Portage groups of Page 135 North America. It seems quite possible, also, that the so-called "Carboniferous Slates" of Ireland correspond with this group, and that the former would be more properly regarded as forming the summit of the Devonian than the base of the Carboniferous.

In no country in the world, probably, is there a finer or more complete exposition of the strata intervening between the Silurian and Carboniferous deposits than in the United States. The following are the main subdivisions of the Devonian rocks in the State of New York, where the series may be regarded as being typically developed (fig. 67):—

(1) Cauda-Galli Grit and Schoharie Grit.—Considering the "Oriskany Sandstone" as the summit of the Upper Silurian, the base of the Devonian is constituted by the arenaceous deposits known by the above names, which rest quite conformably upon the Silurian, and which represent the Lower Devonian of Devonshire. The Cauda-Galli Grit is so called from the abundance of a peculiar spiral fossil (Spirophyton cauda-Galli), which is of common occurrence in the Carboniferous rocks of Britain, and is supposed to be the remains of a sea-weed.

(2) The Corniferous or Upper Helderberg Limestone.—A series of limestones usually charged with considerable quantities of siliceous matter in the shape of hornstone or chert (Lat. cornu, horn). The thickness of this group rarely exceeds 300 feet; but it is replete with fossils, more especially with the remains of corals. The Corniferous Limestone is the equivalent of the coral-bearing limestones of the Middle Devonian of Devonshire and the great "Eifel Limestone" of Germany.

(3) The Hamilton Group—consisting of shales at the base ("Marcellus shales"); flags, shales, and impure limestones ("Hamilton beds") in the middle; and again a series of shales ("Genesee Slates") at the top. The thickness of this group varies from 200 to 1200 feet, and it is richly charged with marine fossils.

(4) The Portage Group.—A great series of shales, flags, and shaly sandstones, with few fossils.

(5) The Chemung Group.—Another great series of sandstones and shales, but with many fossils. The Portage and Chemung groups may be regarded as corresponding with the Upper Devonian of Devonshire. The Chemung beds are succeeded by a great series of red sandstones and shales—the Page 136 "Catskill Group"—which pass conformably upwards into the Carboniferous, and which may perhaps be regarded as the equivalent of the great sandstones of the Upper Old Red in Scotland.

Throughout the entire series of Devonian deposits in North America no unconformability or physical break of any kind has hitherto been detected; nor is there any marked interruption to the current of life, though each subdivision of the series has its own fossils. No completely natural line can thus be indicated, dividing the Devonian in this region from the Silurian on the one hand, and the Carboniferous on the other hand. At the same time, there is the most ample evidence, both stratigraphical and palæontological, as to the complete independence of the American Devonian series as a distinct life-system between the older Silurian and the later Carboniferous. The subjoined section (fig. 76) shows diagrammatically the general succession of the Devonian rocks of North America.

As regards the life of the Devonian period, we are now acquainted with a large and abundant terrestrial flora—this being the first time that we have met with a land vegetation capable of reconstruction in any fulness. By the researches of Gœppert, Unger, Dawson, Carruthers, and other botanists, a knowledge has been acquired of a large number of Devonian plants, only a few of which can be noticed here. As might have been anticipated, the greater number of the vegetable remains of this period have been obtained from such shallow-water deposits as the Old Red Sandstone proper and the Gaspè series of North America, and few traces of plant-life occur in the strictly marine sediments. Apart from numerous remains, mostly of a problematical nature, referred to the comprehensive group of the Sea-weeds, a large number of Ferns have now been recognised, some being, of the ordinary plant-like type (Pecopteris, Neuropteris, Alethopteris, Sphenopteris, &c.), whilst others belong to the gigantic group of the "Tree-ferns" (Psaronius, Caulopteris, &c.) Besides these there is an abundant development of the singular extinct types of the Lepidodendroids, the Sigillarioids, and the Calamites, all of which attained their maximum in the Carboniferous. Of these, the Lepidodendra may be regarded as gigantic, tree-like Club-mosses (Lycopodiaceœ); the Calamites are equally gigantic Horse-tails (Equisetaceœ); and the Sigillarioids, equally huge in size, in some respects hold a position intermediate between the Club-mosses and the Pines (Conifers). The Devonian rocks have Page 137 GENERALIZED SECTION OF THE DEVONIAN ROCKS OF NORTH AMERICA.
Fig. 76.
also yielded traces of many other plants (such as Annularia, Asterophyllites, Cardiocarpon, &c.), which acquire a greater pre-dominance in the Carboniferous period, and which will be spoken of in discussing the structure of the plants of the Coal-measures. Upon the whole, the one plant which may be considered as specially characteristic of the Devonian (though not confined to this series) is the Psilophyton (fig. 77) of Dr Dawson. These singular plants have slender branching stems, with sparse needle-shaped leaves, the young stems being at first coiled up, crosier-fashion, like the young fronds of ferns, whilst the old branches carry numerous spore-cases. The Page 138 stems and branches seem to have attained a height of two or three feet; and they sprang from prostrate "root-stocks" or creeping stems. Upon the whole,
Fig. 77.—Restoration of Psilophyton princeps. Devonian, Canada. (After Dawson.) Principal Dawson is disposed to regard Psilophyton as a "generalised type" of plants intermediate between the Ferns and the Club-mosses. Lastly, the Devonian deposits have yielded the remains of the first actual trees with which we are as yet acquainted. About the nature of some of these (Ormoxylon and Dadoxylon) no doubt can be entertained, since their trunks not only show the concentric rings of growth characteristic of exogenous trees in general, but their woody tissue exhibits under the microscope the "discs" which are characteristic of the wood of the Pines and Firs (see fig. 2). The singular genus Prototaxites, however, which occurs in an older portion of the Devonian series than the above, is not in an absolutely unchallenged position. By Principal Dawson it is regarded as the trunk of an ancient Conifer—the most ancient known; but Mr Carruthers regards it as more probably the stem of a gigantic sea-weed. The trunks of Prototaxites (fig. 78, A) vary from one to three feet in diameter, and exhibit concentric rings of growth; but its woody fibres have not hitherto been clearly demonstrated to possess discs. Before leaving the Devonian vegetation, it may be mentioned that the hornstone or chert so abundant in the Corniferous limestone of North America has been shown to contain the remains of various microscopic plants (Diatoms and Desmids). We find also in the same siliceous material the singular spherical bodies, with radiating spines, which occur so abundantly in the chalk flints, and which are termed Xanthidia. These may be regarded Page 139 as probably the spore-cases of the minute plants known as Desmidiœ.

Fig. 78.—A, Trunk of Prototaxites Logani, eighteen inches in diameter, as seen in the cliff near L'Anse Brehaut, Gaspé; B, Two wood-cells showing spiral fibres and obscure pores, highly magnified. Lower Devonian, Canada. (After Dawson)

The Devonian Protozoans have still to be fully investigated. True Sponges (such as Astrtœospongia, Sphœrospongia, &c.) are not unknown; but by far the commonest representatives of this sub-kingdom in the Devonian strata are Stromatopora and its allies. These singular organisms (fig. 79) are not only very abundant in some of the Devonian limestones—both in the Old World and the New—but they often attain very large dimensions. However much they may differ in minor details, the general structure of these bodies is that of numerous, concentrically-arranged, thin, calcareous laminæ, separated by narrow interspaces, which in turn are crossed by numerous delicate vertical pillars, giving the whole mass a cellular structure, and dividing it into innumerable minute quadrangular compartments. Many of the Devonian Stromatoporœ also exhibit on their surface the rounded openings of canals, which can hardly have served any other purpose than that of permitting the sea-water to gain ready access to every part of the organism.

No true Graptolites have ever been detected in strata of Page 140 of Devonian age; and the whole of this group has become extinguished—unless we refer here the still surviving Dictyonemœ. The Cœlenterates, however,
Fig. 79.—a, Part of the under surface of Stromatopora tuberculata, showing the wrinkled basement membrane and the openings of water-canals, of the natural size; b, Portion of the upper surface of the same, enlarged; c, Vertical section of a fragment, magnified to show the internal structure. Corniferous Limestone, Canada. (Original.) are represented by a vast number of Corals, of beautiful forms and very varied types. The marbles of Devonshire, the Devonian limestones of the Eifel and of France, and the calcareous strata of the Corniferous and Hamilton groups of America, are often replete with the skeletons of these organisms—so much so as to sometimes entitle the rock to be considered as representing an ancient coral-reef. In some instances the Corals have preserved their primitive calcareous composition; and if they are embedded in soft shales, they may weather out of the rock in almost all their original perfection. In other cases, as in the marbles of Devonshire, the matrix is so compact and crystalline that the included corals can only be satisfactorily studied by means of polished sections. In other cases, again, the corals have been more or less completely converted into flint, as in the Corniferous limestone of North America. When this is the case, they often come, by the action of the weather, to stand out from Page 141 the enclosing rock in the boldest relief, exhibiting to the observer the most minute details of their organization. As before, the principal
Fig. 80.—Cystiphyllum vesiculosum, showing a succession of cups produces by budding from the original coral. Of the natural size. Devonian, America and Europe. (Original.)
Fig. 81—Zaphrentis cornicula, of the natural size. Devonian, America. (Original.)

Fig. 82—Heliophyllum exiguum, viewed from in front and behind. Of the natural size. Devonian, Canada. (Original.) representatives of the Corals are still referable to the groups of the Rugosa and Tabulata. Amongst the Rugose group we find a vast number of simple "cup-corals," generally known by the quarrymen as "horns," from their shape. Of Page 142 the many forms of these, the species of Cyathophyllum, Heliophyllum (fig. 82), Zaphrentis (fig. 81), and Cystiphyllum (fig. 80), are perhaps those most abundantly represented—none of these genera, however, except Heliophyllum, being peculiar to the Devonian period. There are also numerous compound Rugose corals, such as species of Eridophyllum, Diphyphyllum, Syringopora, Phillipsastrœa, and some of the forms of Cyathophyllum and Crepidophyllum (fig. 83). Some of these compound corals attain a very large size, and form of
Fig. 83.—Portion of a mass of Crepidophyllum Archiaci, of the natural size. Hamilton Formation, Canada. (After Billings.) themselves regular beds, which have an analogy, at any rate, with existing coral-reefs, though there are grounds for believing that these ancient types differed from the modern reef-builders in being inhabitants of deep water. The "Tabulate Corals" are hardly less abundant in the Devonian rocks than the Rugosa; and being invariably compound, they hardly yield to the latter in the dimensions of the aggregations which they sometimes form.

The commonest, and at the same time the largest, of these are the "honeycomb corals," forming the genus Favosites (figs. 84, 85), which derive both their vernacular and their technical names from their great likeness to masses of petrified honeycomb. The most abundant species are Favosites Gothlandica and F. Hemispherica, both here figured, which form masses sometimes not less than two or three feet in diameter. Whilst Favosites has acquired a popular name by its honey-combed appearance, the resemblance of Michelinia to a fossilised Page 143 wasp's nest with the comb exposed is hardly less striking, and has earned for it a similar recognition from the non-scientific
Fig. 84.—Portion of a mass of Favosites Gothlandica, of the natural size. Upper Silurian and Devonian of Europe and America. (Original.) Billings.
Fig. 85.—Fragment of Favosites hemispherica, of the natural size. Upper Silurian and Devonian of America. (After Billings.) public. In addition to these, there are numerous branching or plant-like Tabulate Corals, often of the most graceful form, which are distinctive of the Devonian in all parts of the world.

The Echinoderms of the Devonian period call for little special notice. Many of the Devonian limestones are "crinoidal;" and the Crinoids are the most abundant and widely-distributed representatives of their class in the deposits of this period.

The Cystideans, with doubtful exceptions, have not been recognised in the Devonian; and their place is taken by the allied group of the "Pentremites," which will be further spoken of as occurring in the Carboniferous rocks. On the other hand, the Star-fishes, Brittle-stars, and Sea-urchins are all continued by types more or less closely allied to those of the preceding Upper Silurian.

Of the remains of Ringed-worms (Annelides), the most numerous and the most interesting are the calcareous envelopes of some small tube-inhabiting species. No one who has visited the seaside can have failed to notice the little spiral tubes of the existing Spirorbis growing attached to shells, or covering the fronds of the commoner Sea weeds (especially Fucus serratus). These tubes are inhabited by a small Annelide, and structures of a similar character occur not uncommonly from the Upper Silurian upwards. In the Devonian rocks, Spirorbis is an extremely common fossil, growing in hundreds attached to the outer surface of corals and shells, and appearing Page 144 in many specific forms (figs. 86 and 87); but almost all the known
Fig. 87.—a, Spirobois omphalodes, natural size and enlarged. Devonian, Europe and America; b, Spirorbis Arkonensis, of the natural size and enlarged; c, The same, with the tube twisted in the reverse direction. Devonian, America. (Original.)
Fig. 88.—a b, Spirorbis laxus, enlarged, Upper Silurian, America; c, Spirorbis spinulifera, of the natural size and enlarged, Devonian, Canada. (After Hall and the Author.) examples are of small size, and are liable to escape a cursory examination.

The Crustaceans of the Devonian are principally Eurypterids and Trilobites. Some of the former attain gigantic dimensions, and the quarrymen in the Scotch Old Red give them the name of "seraphim" from their singular scale-like ornamentation. The Trilobites, though still sufficiently abundant in some localites, have undergone a yet further diminution since the close of the Upper Silurian. In both America and Europe quite a number of generic types have survived from the Silurian, but few or no new ones make their appearance during this period
Fig. 88.—Devonian Trilobites; a, Phacops latifrons, Devonian of Britain, the Continent of Europe, and South America; b, Homalonotus armatus, Europe; c, Phacops (Trimerocephalus) lœvis, Europe; d, Head-shield of Phacops (Portlockia) granulatus, Europe. (After Salter and Burmeister.) in either the Old World or the New. The species, however, are distinct; and the Page 145 principal forms belong to the genera Phacops (fig. 88, a, c, d), Homalonotus (fig. 88, b), Proetus, and Bronteus. The species figured above under the name of Phacops latifrons (fig. 88, a), has an almost world-wide distribution, being found in the Devonian of Britain, Belgium, France, Germany, Russia, Spain, and South America; whilst its place is taken in North America by the closely-allied Phacops rana. In addition to the Trilobites, the Devonian deposits have yielded the remains of a number of the minute Ostracoda, such as Entomis ("Cypridina"), Leperditia, &c., which sometimes occur in vast numbers, as in the so-called "Cypridina Slates" of the German Devonian. There are also a few forms of Phyllopods (Estheria). Taken as a whole, the Crustacean fauna of the Devonian period presents many alliances with that of the Upper Silurian, but has only slight relationships with that of the Lower Carboniferous.

Besides Crustaceans, we meet here for the first time with the remains of air-breathing Articulates, in the shape of Insects. So far, these have only been obtained from the Devonian rocks of North America, and they indicate the existence of at least four generic types, all more or less allied to the existing May-flies (Ephemeridœ). One of these interesting primitive insects, namely, Platephemera antiqua (fig. 89), appears to have measured five inches in expanse of wing;
Fig. 89.—Wing of Platephemera antiqua Devonian, America. (After Dawson.) and another (Xelloneura antiquorum) has attached to its wing the remains of a "stridulating-organ" similar to that possessed by the modern Grasshoppers—the instrument, as Principal Dawson remarks, of "the first music of living things that Geology as yet reveals to us."

Amongst the Mollusca, the Devonian rocks have yielded a great number of the remains of Sea-mosses (Polyzoa). Some of these belong to the ancient type Ptilodictya, which seems to disappear here, or to the allied Clathropora (fig. 90), with its fenestrated and reticulated fronds. We meet also with the graceful and delicate stems of Ceriopora (fig. 91).

The majority of the Devonian Polyzoa belong, however, to the great and important Palæozoic group of the Lace-corals (Fenestella, figs. 92 and 94, Retepora, fig. 93, Polypora, and their allies). In all these forms there is a horny skeleton, of a Page 146 fan-like or funnel-shaped form, which grew attached by its base to some foreign body. The frond consists of slightly-diverging or nearly parallel branches, which are
Fig. 90.—Fragment of Clathropora intertexta, of the natural size and enlarged. Devonian, Canada. (Original.)
Fig. 91.—Fragment of Ceriopora Hamiltonensis, of the natural size and enlarged. Devonian, Canada. (Original.) either united by delicate cross-bars, or which bend alternately from side to side, and become directly united with one another at short intervals—in either case giving origin to numerous oval or oblong perforations, which communicate to the whole
Fig. 92.—Fragment of Fenestella magnifica, of the natural size and enlarged. Devonian, Canada. (Original.)
Fig. 93.—Fragment of Retepora Phillipsi, of the natural size and enlarged. Devonian, Canada. (Original.)

Fig. 94.—Fragment of Fenestella cribrosa, of the natural size and enlarged. Dovonian, Canada. (Original.) plant-like colony a characteristic netted and lace-like appearance. On one of its surfaces—sometimes the internal, sometimes the external—the frond carries a number of minute chambers or Page 147 "cells," which are generally borne in rows on the branches, and of which each originally contained a minute animal.

The Brachiopods still continue to be represented in great force through all the Devonian deposits, though not occurring in the true Old Red Sandstone. Besides such old types as Orthis, Strophomena, Lingula, Athyris, and Rhynchonella, we find some entirely new ones; whilst various types which only commenced their existence in the Upper Silurian, now undergo a great expansion and development. This last is especially the case with the two families of the Spiriferidœ and the Produclidœ. The Spirifers, in particular, are especially characteristic of the Devonian, both in the Old and New Worlds—some of the most typical forms, such as Spirifera mucronata (fig. 96), having the shell "winged," or with the
Fig. 95.—Spirifera sculptilis. Devonian, Canada. (After Billings.)
Fig. 96.—Spirifera mucronata. Devonian, America. (After Billings.) lateral angles prolonged to such an extent as to have earned for them the popular name of "fossil-butterflies." The closely-allied Spirifera disjunda occurs in Britain, France, Spain, Belgium, Germany, Russia, and China. The family of the Productidœ commenced to exist in the Upper Silurian, in the genus Chonetes, and we shall hereafter find it culminating in the Carboniferous in many forms of the great genus Producta[17] itself. In the Devonian period, there is an intermediate state of things, the genus Chonetes being continued in new and varied types, and the Carboniferous Produdœ being represented by many forms of the allied group Productella. Amongst other well-known Devonian Brachiopods may be mentioned the two long-lived and persistent types Atrypa reticularis (fig. 97) and Strophomena rhomboidalis (fig. 98). The former of these commences in the Upper Silurian, but is more abundantly developed in the Devonian, having a geographical range that is nothing less than world-wide; whilst the latter commences in the Lower Silurian, Page 148 and, with an almost equally cosmopolitan range, survives into the Carboniferous period.

[Footnote 17: The name of this genus is often written Productus, just as Spirifera is often given in the masculine gender as Spirifer (the name originally given to it). The masculine termination to these names is, however, grammatically incorrect, as the feminine noun cochlea (shell) is in these cases understood.]

Fig. 97.—Atrypa reticularis. Upper Silurian and Devonian of Europe and America. (After Billings.)

The Bivalves (Lamellibranchiata) of the Devonian call for no special comment, the genera Pterinea and Megalodon being, perhaps, the most noticeable. The Univalves
Fig. 98.—Strophomena rhomboidalis. Lower Silurian, Upper Silurian, and Devonian of Europe and America. (Gasteropods), also, need not be discussed in detail, though many interesting forms of this group are known. The type most abundantly represented, especially in America, is Platyceras (fig. 99), comprising thin, wide-mouthed shells,
Fig. 99.—Different views of Platyceras dumosum, of the natural size. Devonian, Canada. (Original.) probably most nearly allied to the existing "Bonnet-limpets," and sometimes attaining very considerable dimensions. We may also note the continuance of the genus Euomphalus, with its discoidal spiral shell. Amongst the Heteropods, the survival of Bellerophon is to be recorded; and in the "Winged-snails," or Pteropods, we find new forms of the old genera Tentaculites and Conularia Page 149 (fig. 100). The latter, with its fragile, conical, and often beautifully ornamented shell, is especially noticeable.

The remains of Cephalopoda are far from uncommon in the
Fig. 100.—Conularia ornata, of the natural size. Devonian, Europe. Devonian deposits, all the known forms being still Tetrabranchiate. Besides the ancient types Orthoceras and Cyrtoceras, we have now a predominance of the spirally-coiled chambered shells of Goniatites and Clymenia. In the former of these the shell is shaped like that of the Nautilus; but the partitions between the chambers ("septa") are more or less lobed, folded, or angulated, and the "siphuncle" runs along the back or convex side of the shell—these being characters which approximate Goniatites to the true Ammonites of the later rocks. In Clymenia, on the other hand, whilst the shell (fig. 101) is coiled into a flat spiral, and the partitions or septa are simple or only slightly lobed, there is still this difference, as compared with the Nautilus, that the tube of the siphuncle is placed on the inner or concave side of the shell. The
Fig. 101.—Clymenia Sedgwickii. Devonian, Europe. species of Clymenia are exclusively Devonian in Page 150 their range; and some of the limestones of this period in Germany are so richly charged with fossils of this genus as to have received the name of "Clymenien-kalk."

The sub-kingdom of the Vertebrates is still represented by Fishes only; but these are so abundant, and belong to such varied types, that the Devonian period has been appropriately called the "Age of Fishes." Amongst the existing fishes there are three great groups which are of special geological importance, as being more or less extensively represented in past time. These groups are: (1) The Bony Fishes (Teleostei), comprising most existing fishes, in which the skeleton is more or less completely converted into bone; the tail is symmetrically lobed or divided into equal moieties; and the scales are usually thin, horny, flexible plates, which overlap one another to a greater or less extent. (2) The Ganoid Fishes (Ganoidei), comprising the modern Gar-pikes, Sturgeons, &c., in which the skeleton usually more or less completely retains its primitive soft and cartilaginous condition; the tail is generally markedly unsymmetrical, being divided into two unequal lobes; and the scales (when present) have the form of plates of bone, usually covered by a layer of shining enamel. These scales may overlap; or they may be rhomboidal plates, placed edge to edge in oblique rows; or they have the form of large-sized bony plates, which are commonly united in the region of the head to form a regular buckler. (3) The Placoid Fishes, or Elasmobranchii, comprising the Sharks, Rays, and Chimœrœ of the present day, in which the skeleton is cartilaginous; the tail is unsymmetrically lobed; and the scales have the form of detached bony plates of variable size, scattered in the integument.

It is to the two last of these groups that the Devonian fishes belong, and they are more specially referable to the Ganoids. The order of the Ganoid fishes at the present day comprises but some seven or eight genera, the species of which principally or exclusively inhabit fresh waters, and all of which are confined to the northern hemisphere. As compared, therefore, with the Bony fishes, which constitute the great majority of existing forms, the Ganoids form but an extremely small and limited group. It was far otherwise, however, in Devonian times. At this period, the bony fishes are not known to have come into existence at all, and the Ganoids held almost undisputed possession of the waters. To what extent the Devonian Ganoids were confined to fresh waters remains yet to be proved; and that many of them lived in the sea is certain. It was formerly supposed that the Old Red Sandstone of Scotland and Ireland, with its abundant fish-remains, might perhaps be a fresh-water deposit, since the habitat of its fishes is Page 151 uncertain, and it contains no indubitable marine fossils. It has been now shown, however, that the marine Devonian strata of Devonshire and the continent of Europe contain some of the most characteristic of the Old Red Sandstone fishes of Scotland; whilst the undoubted marine deposit of the Corniferous limestone of North America contains numerous shark-like and Ganoid fishes, including such a characteristic Old Red genus as Coccosleus. There can be little doubt, therefore, but that the majority of the Devonian fishes were truly marine in their habits, though it is probable that many of them lived in shallow water, in the immediate neighbourhood of the shore, or in estuaries.

The Devonian Galloids belong to a number of groups; and it is
Fig. 102.—Fishes of the Devonian rocks of America. a, Diagram of the jaws and teeth of Dinichthys Hertzeri, viewed from the front, and greatly reduced; b, Diagram of the skull of Macropetalichthys Sullivanti, reduced in size; c, A portion of the enamelled surface of the skull of the same, magnified; d, One of the scales of Onychodus sigmoides, of the natural size; e, One of the front teeth of the lower jaw of the same, of the natural size: f, Fin-spine of Machœracanthus major, a shark-like fish, reduced in size. (After Newberry.)] only possible to notice a few of the most important forms here. The modern group of the Sturgeons is represented, Page 152 more or less remotely, by a few Devonian fishes—such as Asterosteus; and the great Macropetalichthys of the Corniferous limestone of North America is believed by Newberry to belong to this group. In this fish (fig. 102, b) the skull was of large size, its outer surface being covered with a tuberculated enamel; and, as in the existing Sturgeons, the mouth seems to have been wholly destitute of teeth. Somewhat allied, also, to the Sturgeons, is a singular group of armoured fishes, which is highly characteristic of the Devonian of Britain and Europe, and less so of that of America. In these curious forms the head and front extremity of the body were protected by a buckler composed of large enamelled plates, more or less firmly united to one another; whilst the hinder end of the body was naked, or was protected with small scales. Some forms of this group—such as Pteraspis and Coccosteus—date from the Upper Silurian; but they attain their maximum in the Devonian, and none of them are known to pass upwards into the overlying Carboniferous rocks. Amongst the most characteristic forms of this group may be mentioned Cephalaspis (fig. 103) and Pterichthys (fig. 104). In the former of these the head-shield is of a
Fig. 103.—Cephalaspis Lyellii. Old Red Sandstone, Scotland. (After Page.) crescentic shape, having its hinder angles produced backwards into long "horns," giving it the shape of a "saddler's knife." No teeth have been discovered; but the body was covered with small ganoid scales, and there was an unsymmetrical tail-fin. In Pterichthys—which, like the preceding, was first brought to light by the labours of Hugh Miller—the whole of the head and the front part of the body were defended by a buckler of firmly-united enamelled plates, whilst the rest of the body was covered with small scales. The form of the "pectoral fins" was quite unique—these having the shape of two long, curved spines, somewhat like wings, covered by finely-tuberculated ganoid plates. All the preceding forms Page 153 of this group are of small size; but few fishes, living or extinct, could rival the proportions of the great Dinichthys, referred
Fig. 104.—Pterichthys cornutus. Old Red Sandstone, Scotland. (After Agassiz.) to this family by Newberry. In this huge fish (fig. 102, a) the head alone is over three feet in length, and the body is supposed to have been twenty-five or thirty feet long. The head was protected by a massive cuirass of bony plates firmly articulated together, but the hinder end of the body seems to have been simply enveloped in a leathery skin. The teeth are of the most formidable description, consisting in both jaws of serrated dental plates behind, and in front of enormous conical tusks (fig. 102, a). Though immensely larger, the teeth of Dinichthys present a curious resemblance to those of the existing Mud-fishes (Lepidosiren).

In another great group of Devonian Ganoids, we meet with fishes more or less closely allied to the living Polypteri (fig. 105) of the Nile and Senegal. In this group (fig. 106) the pectoral fins consist of a central scaly lobe carrying the fin-rays on both sides, the scales being sometimes rounded and overlapping (fig. 106), or more commonly rhomboidal and placed edge to edge (fig. 105, A). Numerous forms of these "Fringe-finned" Ganoids occur in the Devonian strata, such as Holoptychius, Glyotolœmus, Osteolepis, Phaneropleuron, &c. To this group is also to be ascribed the huge Onychodus (fig. 102, d and e), with its large, rounded, overlapping scales, an inch in diameter, and its powerful pointed teeth. It is to be remembered, however, that some of these "Fringe-finned" Ganoids are probably referable to the small but singular group of the "Mud-fishes" (Dipnoi), represented at the present day by the singular Lepidosiren of South America and Africa, and the Ceratodus of the rivers of Queensland.

Leaving the Ganoid fishes, it still remains to be noticed that the Devonian deposits have yielded the remains of a number of fishes more or less closely allied to the existing Sharks, Page 154 Rays, and Chimœrœ (the Elasmobranchii). The majority of the forms here alluded to are allied not to the true Sharks and Dog-fishes, but to the more peaceable "Port Jackson
Fig. 105.—A, Polypterus, a recent Ganoid fish; B, Osteolepis, a Devonian Ganoid; a a, Pectoral fins, showing the fin-rays arranged round a central lobe. Sharks," with their blunt teeth, adapted for crushing the shells of Molluscs. The collective name of "Cestracionts" is applied to these; and we have evidence of their past existence in the Devonian seas
Fig. 106.—Holoptychius nobilissimus, restored. Old Red Sandstone, Scotland. A, Scale of the same. both by their teeth, and by the defensive spines which were implanted in front of a greater or less number of the fins. These are bony spines, often variously grooved, serrated, or ornamented, with hollow bases, implanted in the integument, and capable of being erected or depressed at will. Page 155 Many of these "fin-spines" have been preserved to us in the fossil condition, and the Devonian rocks have yielded examples belonging to many genera. As some of the true Sharks and Dog-fishes, some of the Ganoids, and even some Bony Fishes, possess similar defences, it is often a matter of some uncertainty to what group a given spine is to be referred. One of these spines, belonging to the genus Machœracanthus, from the Devonian rocks of America, has been figured in a previous illustration (fig. 102, f).

In conclusion, a very few words may be said as to the validity of the Devonian series as an independent system of rocks, preserving in its successive strata the record of an independent system of life. Some high authorities have been inclined to the view that the Devonian formation has in nature no actual existence, but that it is made up partly of beds which should be referred to the summit of the Upper Silurian, and partly of beds which properly belong to the base of the Carboniferous. This view seems to have been arrived at in consequence of a too exclusive study of the Devonian series of the British Isles, where the physical succession is not wholly clear, and where there is a striking discrepancy between the organic remains of those two members of the series which are known as the "Old Red Sandstone" and the "Devonian" rocks proper. This discrepancy, however, is not complete; and, as we have seen, can be readily explained on the supposition that the one group of rocks presents us with the shallow water and littoral deposits of the period, while in the other we are introduced to the deep-sea accumulations of the same period. Nor can the problem at issue be solved by an appeal to the phenomena of the British area alone, be the testimony of these what it may. As a matter of fact, there is at present no sufficient ground for believing that there is any irreconcilable discordance between the succession of rocks and of life in Britain during the period which elapsed between the deposition of the Upper Ludlow and the formation of the Carboniferous Limestone, and the order of the same phenomena during the same period in other regions. Some of the Devonian types of life, as is the case with all great formations, have descended unchanged from older types; others pass upwards unchanged to the succeeding period: but the fauna and flora of the Devonian period are, as a whole, quite distinct from those of the preceding Silurian or the succeeding Carboniferous; and they correspond to an equally distinct rock-system, which in point of time holds an intermediate position between the two great groups just mentioned. As Page 156 before remarked, this conclusion may be regarded as sufficiently proved even by the phenomena of the British area; but it maybe said to be rendered a certainty by the study of the Devonian deposits of the continent of Europe—or, still more, by the investigation of the vast, for the most part uninterrupted and continuous series of sediments which commenced to be laid down in North America at the beginning of the Upper Silurian, and did not cease till, at any rate, the close of the Carboniferous.

LITERATURE.

The following list comprises the more important works and memoirs to which the student of Devonian rocks and fossils may refer:—