Click here to return to the:  Table of Contents

THE ANCIENT
LIFE-HISTORY
OF THEĀ 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 Page 116 "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.

Page 117 GENERALIZED SECTION OF THE UPPER SILURIAN STRATA OF WALES AND SHROPSHIRE.
Fig. 57.
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 Page 118 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 Page 119 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. 1
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.

Page 120 Amongst the Echinodermata, all those orders which have hard parts capable of ready preservation are more or less largely Fig. 59
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 Page 121 plates of these creatures, and some of them (such as the Wenlock Limestone of Dudley) have yielded Fig. 60
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
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 Page 122 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
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 Page 123 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
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 Page 124 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
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 Page 125 narrow, sword-shaped spine. Fragments of the skeleton are easily recognised by the peculiar scale-like markings with Fig. 65
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 Page 126 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
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 Page 127 Trimerellids; the old and at the same time modern Lingulœ, Discinœ, and Craniœ; together with many species of Atrypa (fig. 68, e), Fig. 68
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.

[Footnote 16: In all the Lamp-shells the mouth is provided with two long fleshy organs, which carry delicate filaments on their sides, and which are usually coiled into a spiral. These organs are known as the "arms," and it is from their presence that the name of "Brachiopoda" is derived (Gr. brachion, arm; podes, feet). In some cases the arms are merely coiled away within the shell, without any support; but in other cases they are carried upon a more or less elaborate shelly loop, often spoken of as the "carriage-spring apparatus." In the Spirifers, and in other ancient genera, this apparatus is coiled up into a complicated spiral (fig. 67). It is these "arms," with or without Fig. 67
Fig. 67.—Spirifera hysterica. The right-hand figure shows the interior of the dorsal valve with the calcareous spires for the support of the arms.
the supporting loops or spires, which serve as one of the special characters distinguishing the Brachiopods from the true Bivalves (Lamellibranchiata).]

Fig. 69
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, Page 128 such as the huge and thick-shelled Megalomi of the American Wenlock formation, the Bivalves (Lamellibranchiata) present little of Fig. 70
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
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 Page 129 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
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
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.

Page 130 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
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
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 Page 131 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.


All contents of www.AgeOfDinosaurs.com are Copyrighted