THE ANCIENT
LIFE-HISTORY
OF THE EARTH
Chapter 10:
THE UPPER SILURIAN PERIOD.
Having now treated of the Lower Silurian period at considerable
length, it will not be necessary to discuss the succeeding group of
the Upper Silurian in the same detail—the more so, as
with a general change of species the Upper Silurian animals
belong for the most part to the same great types as those which
distinguish the Lower Silurian. As compared, also, as regards
the total bulk of strata concerned, the thickness of the Upper
Silurian is generally very much below that of the Lower Silurian,
indicating that they represent a proportionately shorter period of
time. In considering the general succession of the Upper Silurian
beds, we shall, as before, select Wales and America as being two
regions where these deposits are typically developed.
In Wales and its borders the general succession of the Upper
Silurian rocks may be taken to be as follows, in ascending order
(fig. 57):—
(1) The base of the Upper Silurian series is constituted by a
series of arenaceous beds, to which the name of "May Hill Sandstone"
was applied by Sedgwick. These are succeeded by a series of
greenish-grey or pale-grey slates ("Tarannon Shales"), sometimes
of great thickness; and these two groups of beds together form
what may be termed the "May Hill Group" (Upper Llandovery
of Murchison). Though not very extensively developed in Britain,
this zone is one very well marked by its fossils; and it corresponds
with the "Clinton Group" of North America, in which similar fossils
occur. In South Wales this group is clearly unconformable to the
highest member of the subjacent Lower Silurian (the Llandovery
group); and there is reason to believe that a similar, though
less conspicuous, physical break occurs very generally between
the base of the Upper and the summit of the Lower Silurian.
(2) The Wenlock Group succeeds the May Hill group, and
constitutes the middle member of the Upper Silurian. At its base
it may have an irregular limestone ("Woolhope Limestone"), and
its summit may be formed by a similar but thicker calcareous
deposit ("Wenlock Limestone"); but the bulk of the group is made
up of the argillaceous and shaly strata known as the "Wenlock
Shale." In North Wales the Wenlock group is, represented by a
great accumulation of flaggy and gritty strata (the "Denbighshire
Flags and Grits"), and similar beds (the
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"Coniston Flags" and "Coniston Grits") take the same place in
the north of England.
(3) The Ludlow Group is the highest member of the Upper
Silurian, and consists typically of a lower arenaceous and shaly
series (the "Lower Ludlow Rock") a middle calcareous member (the
"Aymestry Limestone"), and an upper shaly and sandy series (the
"Upper Ludlow Rock" and "Downton Sandstone"). At the summit, or
close to the summit, of the Upper Ludlow, is a singular stratum
only a few inches thick (varying from an inch to a foot), which
contains numerous remains of crustaceans and fishes, and is well
known under the name of the "bone-bed." Finally, the Upper Ludlow
rock graduates invariably into a series of red sandy deposits,
which, when of a flaggy character, are known locally as the
"Tile-stones." These beds are probably to be regarded as the
highest member of the Upper Silurian; but they are sometimes
looked upon as passage-beds into the Old Red Sandstone, or as
the base of this formation. It is, in fact, apparently impossible
to draw any actual line of demarcation between the Upper Silurian
and the overlying deposits of the Devonian or Old Red Sandstone
series. Both in Britain and in America the Lower Devonian beds
repose with perfect conformity upon the highest Silurian beds, and
the two formations appear to pass into one another by a gradual
and imperceptible transition.
The Upper Silurian strata of Britain vary from perhaps 3000 or
4000 feet in thickness up to 8000 or 10,000 feet. In North America
the corresponding series, though also variable, is generally of
much smaller thickness, and may be under 1000 feet. The general
succession of the Upper Silurian deposits of North America is
as follows:—
(1) Medina Sandstone.—This constitutes the base of the
Upper Silurian, and consists of sandy strata, singularly devoid
of life, and passing below in some localities into a conglomerate
("Oneida Conglomerate"), which is stated to contain pebbles derived
from the older beds, and which would thus indicate an unconformity
between the Upper and Lower Silurian.
(2) Clinton Group.—Above the Medina sandstone are beds
of sandstone and shale, sometimes with calcareous bands, which
constitute what is known as the "Clinton Group." The Medina and
Clinton groups are undoubtedly the equivalent of the "May Hill
Group" of Britain, as shown by the identity of their fossils.
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GENERALIZED SECTION OF THE UPPER SILURIAN STRATA OF WALES AND
SHROPSHIRE.
Fig. 57.
(3) Niagara Group.—This group consists typically of a
series of argillaceous beds ("Niagara Shale") capped by limestones
("Niagara Limestone"); and the name of the group is derived from
the fact that it is over limestones of this age that the Niagara
river is precipitated to form the great Falls. In places the
Niagara group is wholly calcareous, and it is continued upwards
into a series of marls and sandstones, with beds of salt and masses
of gypsum (the "Salina Group"), or into a series of magnesian
limestones ("Guelph Limestones"). The Niagara group, as a whole,
corresponds unequivocally with the Wenlock group of Britain.
(4) Lower Helderberg Group.—The Upper Silurian period
in North America was terminated by the deposition of a series of
calcareous beds, which derive the name of "Lower Helderberg" from
the Helderberg mountains, south of Albany, and
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which are divided into several zones, capable of recognition
by their fossils, and known by local names (Tentaculite Limestone,
Water-lime, Lower Pentamerus Limestone, Delthyris Shaly Limestone,
and Upper Pentamerus Limestone). As a whole, this series may be
regarded as the equivalent of the Ludlow group of Britain, though
it is difficult to establish any precise parallelism. The summit
of the Lower Heiderberg group is constituted by a coarse-grained
sandstone (the "Oriskany Sandstone"), replete with organic
remains, which have to a large extent a Silurian facies.
Opinions differ as to whether this sandstone is to be regarded
as the highest bed of the Upper Silurian or the base of the
Devonian. We thus see that in America, as in Britain, no other
line than an artificial one can be drawn between the Upper
Silurian and the overlying Devonian.
As regards the life of the Upper Silurian period, we have,
as before, a number of so-called "Fucoids," the true vegetable
nature of which is in many instances beyond doubt. In addition
to these, however, we meet for the first time, in deposits of
this age, with the remains of genuine land-plants, though our
knowledge of these is still too scanty to enable us to construct
any detailed picture of the terrestrial vegetation of the period.
Some of these remains indicate the existence of the remarkable
genus Lepidodendron—a genus which played a part of
great importance in the forests of the Devonian and Carboniferous
periods, and which may be regarded as a gigantic and extinct type of
the Club-mosses (Lycopodiaceœ). Near the summit of the
Ludlow formation in Britain there have also been found beds charged
with numerous small globular bodies, which Dr Hooker has shown
to be the seed-vessels or "sporangia" of Club-mosses. Principal
Dawson further states that he has seen in the same formation
fragments of wood with the structure of the singular Devonian
Conifer known as Prototaxites. Lastly, the same distinguished
observer has described from the Upper Silurian of North America
the remains of the singular land-plants belonging to the genus
Psilophyton, which will be referred to at greater length
hereafter.
The marine life of the Upper Silurian is in the main constituted by
types of animals similar to those characterising the Lower Silurian,
though for the most part belonging to different species. The
Protozoans are represented principally by Stromatopora
and Ischadites, along with a number of undoubted sponges
(such as Amphispongia, Astrœospongia, Astylospongia,
and Palœomanon).
Amongst the Cœlenterates, we find the old group of
Graptolites now verging on extinction. Individuals still
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remain numerous, but the variety of generic
and specific types has now become greatly reduced. All the
branching and complex forms of the Arenig, the twin-Graptolites
Fig. 58.—A, Monograptus priodon, slightly enlarged.
B, Fragment of the same viewed from behind. C, Fragment of the
same viewed in front, showing the mouths of the cellules. D,
Cross-section of the same. From the Wenlock Group (Coniston
Flags of the North of England). (Original.)
and Dicranograpti of the Llandeilo, and the double-celled
Diplograpti and Climacograpti of the Bala group,
have now disappeared. In their place we have the singular
Retiolites, with its curiously-reticulated skeleton; and
several species of the single-celled genus Monograptus,
of which a characteristic species (M. Priodon) is here
figured. If we remove from this group the plant-like
Dictyonemœ, which are still present, and which
survive into the Devonian, no known species of Graptolite
has hitherto been detected in strata higher in geological position
than the Ludlow. This, therefore, presents us with the first
instance we have as yet met with of the total disappearance and
extinction of a great and important series of organic forms.
The Corals are very numerously represented in the Upper
Silurian rocks some of the limestones (such as the Wenlock Limestone)
being often largely composed of the skeletons of these animals.
Almost all the known forms of this period belong to the two great
divisions of the Rugose and Tabulate corals, the former being
represented by species of Zaphrentis, Omphyma, Cystiphyllum,
Strombodes, Acervularia, Cyathophyllum, &c.; whilst the latter
belong principally to the genera Favosites, Chœtetes,
Halysites, Syringopora, Heliolites, and Plasmopora.
Amongst the Rugosa, the first appearance of the great and
important genus Cyathophyllum, so characteristic of the
Palæozoic period, is to be noted; and amongst the
Tabulata we have similarly the first appearance, in force
at any rate, of the widely-spread genus Favosites—the
"Honeycomb-corals." The "Chain-corals" (Halysites), figured
below (fig. 59), are also very common examples of the Tabulate
corals during this period, though they occur likewise in the Lower
Silurian.
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Amongst the Echinodermata, all those orders which have hard
parts capable of ready preservation are more or less largely
Fig. 59.—a, Halysites catenularia, small variety, of
the natural size; b, Fragment of a large variety of the
same, of the natural size; c, Fragment of limestone with
the tubes of Halysites agglomerata, of the natural size;
d, Vertical section of two tubes of the same, showing the
tabulæ, enlarged. Niagara Limestone (Wenlock), Canada.
(Original.)
represented. We have no trace of the Holothurians or Sea-cucumbers;
but this is not surprising, as the record of the past is throughout
almost silent as to the former existence of these soft-bodied
creatures, the scattered plates and spicules in their skin offering
a very uncertain chance of preservation in the fossil condition.
The Sea-urchins (Echinoids) are said to be represented by
examples of the old genus Palœchinus. The Star-fishes
(Asteroids) and the Brittle-stars (Ophiuroids) are,
comparatively speaking, largely represented; the former by species
of Palasterina (fig. 60), Palœaster (fig. 60),
Palœocoma (fig. 60), Petraster, Glyptaster, and
Lepidaster—and the latter by species of Protaster
(fig. 61), Palœodiscus, Acroura, and Eucladia.
The singular Cystideans, or "Globe Crinoids," with their
globular or ovate, tesselated bodies (fig. 46, A, C, D,), are also
not uncommon in the Upper Silurian; and if they do not become finally
extinct here, they certainly survive the close of this period by
but a very brief time. By far the most important, however, of the
Upper Silurian Echinodenns, are the Sea-lilies or Crinoids.
The limestones of this period are often largely composed of the
fragmentary columns and detached
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plates of these creatures, and some
of them (such as the Wenlock Limestone of Dudley) have yielded
Fig. 60.—Upper Silurian Star-fishes. 1, Palasterina
primœva, Lower Ludlow; 2, Paloeaster Ruthveni,
Lower Ludlow; 3, Palœocoma Colvini, Lower Ludlow.
(After Salter.)
perhaps the most exquisitely-preserved examples of this group
with which we are as yet acquainted. However varied in their
forms, these beautiful organisms consist of a globular, ovate, or
Fig. 61.—A, Protaster Sedgwickii, showing the disc and
bases of the arms; B, Portion of an arm, greatly enlarged.
Lower Ludlow. (After Salter.)
pear-shaped body (the "calyx"), supported upon a longer or shorter
jointed stem (or "column"). The body is covered externally with
an armour of closely-fitting calcareous plates (fig. 62), and
its upper surface is protected by similar but smaller plates
more loosely connected by a leathery integument. From the upper
surface of the body, round its margin, springs a series of longer
or shorter flexible processes, composed of innumerable calcareous
joints or pieces, movably united with one
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another. The arms are
typically five in number; but they generally subdivide at least
once, sometimes twice, and they are furnished with similar but
Fig. 62.—Upper Silurian Crinoids. a, Calyx and arms
of Eucalyptocrinus polydactylus, Wenlock Limestone; b,
Ichthyocrinus lœvis, Niagara Limestone, America; c,
Taxocrinus tuberculatus, Wenlock Limestone. (After M'Coy
and Hall.)
more slender lateral branches or "pinnules," thus giving rise
to a crown of delicate feathery plumes. The "column" is the stem
by which the animal is attached permanently to the bottom of the
sea; and it is composed of numerous separate plates, so jointed
together that whilst the amount of movement between any two pieces
must be very limited, the entire column acquires more or less
flexibility, allowing the organism as a whole to wave backwards
and forwards on its stalk. Into the exquisite minutiœ
of structure by which the innumerable parts entering into the
composition of a single Crinoid are adapted for their proper
purposes in the economy of the animal, it is impossible to enter
here. No period, as before said, has yielded examples of greater
beauty than the Upper Silurian, the principal genera represented
being Cyathocrinus, Platycrinus, Marsupiocrinus, Taxocrinus,
Eucalyptocrinus, Ichthyocrinus, Mariacrinus, Periechocrinus,
Glyptocrinus, Crotalocrinus, and Edriocrinus.
The tracks and burrows of Annelides are as abundant in
the Upper Silurian strata as in older deposits, and have just
as commonly been regarded as plants. The most abundant forms
are the cylindrical, twisted bodies (Planolites), which are
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so frequently found on the surfaces of sandy
beds, and which have been described as the stems of sea-weeds.
These fossils (fig. 63), however, can be nothing more, in most
Fig. 63.—Planolites vulgaris, the filled-up burrows
of a marine worm. Upper Silurian (Clinton Group), Canada.
(Original.)
cases, than the filled-up
burrows of marine worms resembling the living Lob-worms. There
are also various remains which belong to the group of the
tube-inhabiting Annelides (Tubicola). Of this nature are
the tubes of Serpulites and Cornultites, and the
little spiral discs of Spirorbis Lewisii.
Amongst the Articulates, we still meet only with the remains
of Crustaceans. Besides the little bivalved
Ostracoda—which here are occasionally found of the size
of beans—and various Phyllopods of different kinds, we
have an abundance of Trilobites. These last-mentioned ancient
types, however, are now beginning to show signs of decadence; and
though still individually numerous, there is a great diminution
in the number of generic types. Many of the old genera, which
flourished so abundantly in Lower Silurian seas, have now died out;
and the group is represented chiefly by species of Cheirurus,
Encrinurus, Harpes, Proetus, Lichas, Acidaspis, Illœnus,
Calymene, Homalonotus, and Phacops—the last of
these, one of the
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highest and most beautiful of the groups of Trilobites, attaining
here its maximum of development. In the annexed illustration
(fig. 64) some of the characteristic Upper Silurian Trilobites are
Fig. 64.—Upper Silurian Trilobites. a, Cheirurus
bimucronatus, Wenlock and Caradoc; b, Phacops
longicaudatus, Wenlock, Britain, and America; c, Phacops
Downingiœ, Wenlock and Ludlow; d, Harpes ungula,
Upper Silurian, Bohemia. (After Salter and Barrande.)
represented—all, however, belonging to genera which have their
commencement in the Lower Silurian period. In addition to the above,
the Ludlow rocks of Britain and the Lower Helderberg beds of North
America have yielded the remains of certain singular Crustaceans
belonging to the extinct order of the Eurypterida. Some
of these wonderful forms are not remarkable for their size; but
others, such as Pterygotus Anglicus (fig. 65), attain a
length of six feet or more, and may fairly be considered as the
giants of their class. The Eurypterids are most nearly allied
to the existing King-crabs (Limuli), and have the anterior
end of the body covered with a great head-shield, carrying two
pairs of eyes, the one simple and the other compound. The feelers
are converted into pincers, whilst the last pair of limbs have
their bases covered with spiny teeth so as to act as jaws, and
are flattened and widened out towards their extremities so as
to officiate as swimming-paddles. The hinder extremity of the
body is composed of thirteen rings, which have no legs attached
to them; and the last segment of the tail is either a flattened
plate or a
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narrow, sword-shaped spine. Fragments of the skeleton
are easily recognised by the peculiar scale-like markings with
Fig. 65.—Pterygotus Anglicus, viewed
from the under side, reduced in size, and restored. c c,
The feelers (antennæ), terminating in nipping-claws; o
o, Eyes; m m, Three pairs of jointed limbs, with
pointed extremities; n n, Swimming-paddles, the bases of
which are spiny and act as jaws. Upper Silurian, Lanarkshire.
(After Henry Woodward.)
which the surface is adorned, and which look not at all unlike
the scales of a fish. The most famous locality for these great
Crustaceans is Lesmahagow, in Lanarkshire, where many different
species have been found. The true King-crabs (Limuli) of
existing seas also appear to have been represented by at least
one form (Neolimulus) in the Upper Silurian.
Coming to the Mollusca, we note the occurrence of the same
great groups as in the Lower Silurian. Amongst the Sea-mosses
(Polyzoa), we have the ancient Lace-corals (Fenestella
and Retepora), with the nearly-allied Glauconome,
and species of Ptilodictya (fig. 66); whilst many forms
often referred here may probably have to be transferred to the
Corals, just as some so-called Corals will ultimately be removed
to the present group.
The Brachiopods continued to flourish during the Upper Silurian
Period in immense numbers and under a greatly increased variety
of forms. The three prominent Lower Silurian genera Orthis,
Strophomena, and Leptœna are still well represented,
though they have lost their former preeminence. Amongst the numerous
types which have now come upon the scene for the first time, or
which have now a special development, are Spirifera and
Pentamerus. In the first of these (fig. 69. b, c),
one of the valves of the shell (the dorsal) is furnished in its
interior with a pair of great calcareous spires, which served
for the support of the long and fringed fleshy processes or "arms"
which were attached to the sides of the mouth.[16] In the genus
Pentamerus (fig. 70) the
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shell is curiously subdivided in its
interior by calcareous plates. The Pentameri commenced their
existence at the very close of the Lower Silurian (Llandovery), and
Fig. 66.—Upper Silurian Polyzoa. 1, Fan-shaped
frond of Rhinopora verrucosa; 1a, Portion of the
surface of the same, enlarged; 2 and 2a, Phœnopora
ensiformis, of the natural size and enlarged; 3 and 3a,
Helopora fragilis, of the natural size and enlarged; 4
and 4a, Ptilodictya raripora, of the natural size
and enlarged. The specimens are all from the Clinton Formation
(May Hill Group) of Canada. (Original.)
survived to the close of the Upper Silurian; but they are specially
characteristic of the May Hill and Wenlock groups, both in Britain
and in other regions. One species, Pentamerus galeatus, is
common to Sweden, Britain, and America. Amongst the remaining Upper
Silurian Brachiopods are the extraordinary
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Trimerellids; the old and at the same time modern
Lingulœ, Discinœ, and Craniœ;
together with many species of Atrypa (fig. 68, e),
Fig. 68.—Upper
Silurian Brachiopods. a a', Leptocœlia
plano-convexa, Clinton Group, America; b b',
Rhynchonella neglecta, Clinton Group, America; c,
Rhynchonella cuneata, Niagara Group, America, and Wenlock
Group, Britain; d d', Orthis elelgantula, Llandeilo to
Ludlow, America and Europe; e e', Atrypa hemispherica,
Clinton Group, America, and Llandovery and May Hill Groups,
Britain; f f', Atrypa congesta, Clinton Group, America;
g g', Orthis Davidsoni, Clinton Group, America. (After
Hall, Billings, and the Author.)
Leptocœlia (fig. 68, a),
Rhynchonella (fig. 68, b, c), Meristella
(fig. 69, a, e, f), Athyris, Retzia, Chonetes,
&c.
|
|
Fig. 69.—a a', Meristella intermedia, Niagara Group, America;
b, Spirifera Niagarensis, Niagara Group, America; c c',
Spirifera crispa, May Hill to Ludlow, Britain, and Niagara
Group, America; d, Strophomena (Streptorhynchus) subplana,
Niagara Group, America; e, Meristella naviformis, Niagara
Group, America; f, Meristella cylindrica, Niagara Group,
America. (After Hall, Billings, and the Author.)
|
The higher groups of the Mollusca are also largely represented
in the Upper Silurian. Apart from some singular types,
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such as the
huge and thick-shelled Megalomi of the American Wenlock
formation, the Bivalves (Lamellibranchiata) present little of
Fig. 70.—Pentamerus Knightii. Wenlock and Ludlow. The
right-hand figure shows the internal partitions of the shell.
special interest; for though sufficiently numerous, they are rarely
well preserved, and their true affinities are often uncertain.
Amongst the most characteristic genera of this period may be
mentioned Cardiola (fig. 71, A and C) and Pterinea
Fig. 71.—Upper Silurian Bivalves. A, Cardiola
interrupta, Wenlock and Ludlow; B, Pterinea subfalcata,
Wenlock; C, Cardiola fibrosa, Ludlow. (After Salter and
M'Coy.)
(fig. 71, B), though the latter survives to a much later date.
The Univalves (Gasteropoda) are very numerous, and a
few characteristic forms are here figured (fig. 72). Of these,
no genus is perhaps more characteristic than Euomphalus
(fig. 72, b), with its flat discoidal shell, coiled up
into an oblique spiral, and deeply hollowed out on one side;
but examples of this group are both of older and of more modern
date. Another very extensive genus, especially in America, is
Platyceras (fig. 72, a and f), with its thin fragile
shell—often hardly coiled up at all—its minute spire,
and its widely-expanded, often sinuated mouth. The British
Acroculiœ should probably be placed here, and the
group has with reason been regarded as allied to the Violet-snails
(Ianthina) of the open Atlantic. The
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species of Platyostoma (fig. 72, h) also belong
to the same family; and the entire group is continued throughout
the Devonian into the Carboniferous. Amongst other well-known
Upper Silurian Gasteropods are species of the genera Holopea
(fig. 72, g), Holopella (fig. 72. e),
Fig. 72.—Upper Silurian Gasteropods. a, Platyceras
ventricosum, Lower Helderberg, America; b, Euomphalus
discors, Wenlock, Britain; c, Holopella obsoleta Ludlow,
Britain; d, Platyschisma helicites, Upper Ludlow, Britain;
e, Holopella gracilior, Wenlock, Britain; f, Platyceras
multisinuatum, Lower Helderberg, America; g, Holopea
subconica, Lower Helderberg, America; h, h', Platyostoma
Niagarense, Niagara Group, America. (After Hall, M'Coy, and
Salter.)
Platyschisma (fig. 72, d), Cyclonema,
Pleurotomaria, Murchisonia, Trochonema, &c. The oceanic
Fig. 73.—Tentaculites ornatus. Upper Silurian of Europe
and North America.
Univalves (Heteropods) are represented mainly by species of
Bellerophon; and the Winged Snails, or Pteropods, can
still boast of the gigantic Thecœ and
Conulariœ, which characterise yet older deposits. The
commonest genus of Pteropoda, however, is Tentaculites
(fig. 73), which clearly belongs here, though it has commonly been
regarded as the tube of an Annelide. The shell in this group
is a conical tube, usually adorned with prominent transverse
rings, and often with finer transverse or longitudinal striæ
as well; and many beds of the Upper Silurian exhibit myriads of
such tubes scattered promiscuously over their surfaces.
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The last and highest group of the Mollusca—that
of the Cephalopoda—is still represented only by
Tetrabranchiate forms; but the abundance and variety of
these is almost beyond belief. Many hundreds of different species
are known, chiefly belonging to the straight Orthoceratites,
but the slightly-curved Cyrtoceras is only little less
common. There are also numerous forms of the genera Phragmoceras,
Ascoceras, Gyroteras, Lituites, and Nautilus. Here, also,
are the first-known species of the genus Goniatites—a
group which attains considerable importance in later deposits, and
which is to be regarded as the precursor of the Ammonites of
the Secondary period.
Finally, we find ourselves for the first time called upon to
consider the remains of undoubted vertebrate animals, in the
Fig. 74.—Head-shield of Pteraspis Banksii, Ludlow
rocks. (After Murchison.)
form of Fishes. The oldest of these remains, so far as
yet known, are found in the Lower Ludlow rocks, and they consist
of the bony head-shields or bucklers of certain singular armoured
fishes belonging to the group of the Ganoids, represented at
the present day by the Sturgeons, the Gar-pikes of North America,
and a few other less familiar forms. The principal Upper Silurian
genus of these is Pteraspis, and the annexed illustration
(fig. 74) will give some idea of the extraordinary form of the
shield covering the head in these ancient fishes. The remarkable
stratum near the top of the Ludlow formation known as the "bone-bed"
has also yielded the remains of shark-like fishes. Some of these,
for which the name of Onchus has been proposed, are in
the form of compressed, slightly-curved spines (fig. 75, A),
which would appear to be of the nature of the strong defensive
spines implanted in front of certain of the fins in many living
fishes. Besides these, have been found fragments of prickly skin
Fig. 75.—A, Spine of Onchus tenuistriatus; B,
Shagreen-scales of Thelodus. Both from the "bone-bed"
of the Upper Ludlow rocks. (After Murchison.)
or shagreen (Sphagodus), along with minute cushion-shaped
bodies (Thelodus, fig. 75, B), which
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are doubtless the bony scales of some fish resembling the modern
Dog-fishes. As the above mentioned remains belong to two distinct,
and at the same time highly-organised, groups of the fishes, it
is hardly likely that we are really presented here with the first
examples of this great class. On the contrary, whether the
so-called "Conodonts" should prove to be the teeth of fishes or
not, we are justified in expecting that unequivocal remains of
this group of animals will still be found in the Lower Silurian.
It is interesting, also, to note that the first appearance of
fishes—the lowest class of vertebrate animals—so far
as known to us at present, does not take place until after all
the great sub-kingdoms of invertebrates have been long in
existence; and there is no reason for thinking that future
discoveries will materially affect the relative order of
succession thus indicated.
LITERATURE.
From the vast and daily-increasing mass of Silurian literature, it
is impossible to do more than select a small number of works which
have a classical and historical interest to the English-speaking
geologist, or which embody researches on special groups of Silurian
animals—anything like an enumeration of all the works and
papers on this subject being wholly out of the question. Apart,
therefore, from numerous and in many cases extremely important
memoirs, by various well-known observers, both at home and abroad,
the following are some of the more weighty works to which the
student may refer in investigating the physical characters and
succession of the Silurian strata and their fossil contents:—
| (1) |
'Siluria.' Sir Roderick Murchison. |
| (2) |
'Geology of Russia in Europe.' Murchison (with M. de Verneuil
and Count von Keyserling). |
| (3) |
'Bassin Silurien de Bohême Centrale.' Barrande. |
| (4) |
'Introduction to the Catalogue of British Palæozoic
Fossils in the Woodwardian Museum of Cambridge.'
Sedgwick. |
| (5) |
'Die Urwelt Russlands.' Eichwald. |
| (6) |
'Report on the Geology of Londonderry, Tyrone,' &c.
Portlock. |
| (7) |
"Geology of North Wales"—'Mem. Geol. Survey of Great
Britain,' vol. iii. Ramsay. |
| (8) |
'Geology of Canada,' 1863. Sir W. E. Logan; and the
'Reports of Progress of the Geological Survey' since
1863. |
| (9) |
'Memoirs of the Geological Survey of Great
Britain.' |
| (10) |
'Reports of the Geological Surveys of the States of New
York, Illinois, Ohio, Iowa, Michigan, Vermont, Wisconsin,
Minnesota,' &c. By Emmons, Hall, Worthen, Meek, Newberry,
Orton, Winchell, Dale Owen, &c. |
| (11) |
'Thesaurus Siluricus.' Bigsby. |
| (12) |
'British Palæozoic Fossils.' M'Coy. |
| (13) |
'Synopsis of the Silurian Fossils of Ireland,'
M'Coy. |
| (14) |
"Appendix to the Geology of North Wales"—'Mem. Geol.
Survey,' vol. iii. Salter. |
|
Page 132
(15) |
'Catalogue of the Cambrian and Silurian Fossils in the
Woodwardian Museum of Cambridge.' Salter. |
| (16) |
'Characteristic British Fossils.' Baily. |
| (17) |
'Catalogue of British Fossils.' Morris. |
| (18) |
'Palæozoic Fossils of Canada.' Billings. |
| (19) |
'Decades of the Geological Survey of Canada.' Billings,
Salter, Rupert Jones. |
| (20) |
'Decades of the Geological Survey of Great Britain.' Salter,
Edward, Forbes. |
| (21) |
'Palæontology of New York,' vols. i.-iii.
Hall. |
| (22) |
'Palæontology of Illinois.' Meek and
Worthen. |
| (23) |
'Palæontology of Ohio.' Meek, Hall, Whitfield,
Nicholson. |
| (24) |
'Silurian Fauna of West Tennessee' (Silurische Fauna des
Westlichen Tennessee). Ferdinand Rœmer. |
| (25) |
'Reports on the State Cabinet of New York.'
Hall. |
| (26) |
'Lethæa Geognostica.' Bronn. |
| (27) |
'Index Palæontologicus.' Bronn. |
| (28) |
'Lethæa Rossica.' Eichwald. |
| (29) |
'Lethæa Suecica.' Hisinger. |
| (30) |
'Palæontologica Suecica.' Angelin. |
| (31) |
'Petrefacta Germaniæ.' Goldfuss. |
| (32) |
'Versteinerungen der Grauwacken-Formation in Sachsen.'
Geinitz. |
| (33) |
'Organisation of Trilobites' (Ray Society).
Burmeister. |
| (34) |
'Monograph of the British Trilobites'
(Palæontographical Society). Salter. |
| (35) |
'Monograph of the British Merostomata'
(Palæontographical Society). Henry Woodward. |
| (36) |
'Monograph of British Brachiopoda' (Palæontographical
Society). Thomas Davidson. |
| (37) |
'Graptolites of the Quebec Group.' James Hall. |
| (38) |
'Monograph of the British Graptolitidæ.'
Nicholson. |
| (39) |
'Monographs on the Trilobites. Pteropods, Cephalopods,
Graptolites,' &c. Extracted from the 'Système Silurien
du Centre de la Bohême.' Barrande. |
| (40) |
'Polypiers Fossiles des Terrains Paleozoiques,' and 'Monograph
of the British Corals' (Palæontographical Society). Milne
Edwards and Jules Haime. |
|
