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 (_Lycopodiaceoe_). 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. Princ.i.p.al 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 const.i.tuted by types of animals similar to those characterising the Lower Silurian, though for the most part belonging to different species.

The _Protozoans_ are represented princ.i.p.ally by _Stromatopora_ and _Ischadites_, along with a number of undoubted sponges (such as _Amphispongia, Astroeospongia, Astylospongia_, and _Paloeomanon_).

Amongst the _Coelenterates_, we find the old group of _Graptolites_ now verging on extinction. Individuals still 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 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 _Dictyonemoe_, which are still present, and which survive into the Devonian, no known species of _Graptolite_ has. .h.i.therto 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.

[Ill.u.s.tration: 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.)]

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 princ.i.p.ally to the genera _Favosites, Choetetes, Halysites, Syringopora, Heliolites_, and _Plasmopora_. Amongst the _Rugosa_, the first appearance of the great and important genus _Cyathophyllum_, so characteristic of the Palaeozoic 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.

[Ill.u.s.tration: 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 tabulae, enlarged.

Niagara Limestone (Wenlock), Canada. (Original.)]

[Ill.u.s.tration: Fig. 60.--Upper Silurian Star-fishes. 1, _Palasterina primoeva_, Lower Ludlow; 2, _Paloeaster Ruthveni_, Lower Ludlow; 3, _Paloeocoma Colvini_, Lower Ludlow. (After Salter.)]

[Ill.u.s.tration: Fig. 61.--A, _Protaster Sedgwickii_, showing the disc and bases of the arms; B, Portion of an arm, greatly enlarged.

Lower Ludlow. (After Salter.)]

Amongst the _Echinodermata_, all those orders which have hard parts capable of ready preservation are more or less largely represented.

We have no trace of the Holothurians or Sea-cuc.u.mbers; 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 _Paloechinus_. The Star-fishes (_Asteroids_) and the Brittle-stars (_Ophiuroids_) are, comparatively speaking, largely represented; the former by species of _Palasterina_ (fig.

60), _Paloeaster_ (fig. 60), _Paloeocoma_ (fig. 60), _Petraster, Glyptaster_, and _Lepidaster_--and the latter by species of _Protaster_ (fig. 61), _Paloeodiscus, 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 plates of these creatures, and some of them (such as the Wenlock Limestone of Dudley) have yielded 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 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 another. The arms are typically five in number; but they generally subdivide at least once, sometimes twice, and they are furnished with similar but 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 _minutioe_ 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 princ.i.p.al genera represented being _Cyathocrinus, Platycrinus, Marsupiocrinus, Taxocrinus, Eucalyptocrinus, Ichthyocrinus, Mariacrinus, Periechocrinus, Glyptocrinus, Crotalocrinus_, and _Edriocrinus_.

[Ill.u.s.tration: Fig 62.--Upper Silurian Crinoids. a, Calyx and arms of _Eucalyptocrinus polydactylus_, Wenlock Limestone; b, _Ichthyocrinus loevis_, Niagara Limestone, America; c, _Taxocrinus tuberculatus_, Wenlock Limestone. (After M'Coy and Hall.)]

[Ill.u.s.tration: Fig. 63.--_Planolites vulgaris_, the filled-up burrows of a marine worm. Upper Silurian (Clinton Group), Canada.

(Original.)]

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 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 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 _Cornult.i.tes_, and the little spiral discs of _Spirorbis Lewisii_.

[Ill.u.s.tration: Fig. 64.--Upper Silurian Trilobites. a, _Cheirurus bimucronatus_, Wenlock and Caradoc; b, _Phacops longicaudatus_, Wenlock, Britain, and America; c, _Phacops Downingioe_, Wenlock and Ludlow; d, _Harpes ungula_, Upper Silurian, Bohemia. (After Salter and Barrande.)]

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, Illoenus, Calymene, Homalonotus_, and _Phacops_--the last of these, one of the highest and most beautiful of the groups of Trilobites, attaining here its maximum of development. In the annexed ill.u.s.tration (fig. 64) some of the characteristic Upper Silurian Trilobites are 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 cla.s.s. 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 narrow, sword-shaped spine. Fragments of the skeleton are easily recognised by the peculiar scale-like markings with 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.

[Ill.u.s.tration: Fig. 65.--_Pterygotus Anglicus_, viewed from the under side, reduced in size, and restored. c c, The feelers (antennae), 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.)]

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.

[Ill.u.s.tration: Fig. 66.--Upper Silurian Polyzoa. 1, Fan-shaped frond of _Rhinopora verrucosa_; 1a, Portion of the surface of the same, enlarged; 2 and 2a, _Phoenopora 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.)]

[Ill.u.s.tration: 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.]

[Ill.u.s.tration: Fig. 68.--Upper Silurian Brachiopods. a a', _Leptocoelia 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.)]

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 _Leptoena_ 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 sh.e.l.l (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 sh.e.l.l is curiously subdivided in its interior by calcareous plates. The _Pentameri_ commenced their existence at the very close of the Lower Silurian (Llandovery), and 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 _Trimerellids_; the old and at the same time modern _Linguloe, Discinoe_, and _Cranioe_; together with many species of _Atrypa_ (fig. 68, e), _Leptocoelia_ (fig. 68, a), _Rhynchonella_ (fig. 68, b, c), _Meristella_ (fig. 69, a, e, f), _Athyris, Retzia, Chonetes_, &c.

[Footnote 16: In all the Lamp-sh.e.l.ls 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 sh.e.l.l, without any support; but in other cases they are carried upon a more or less elaborate sh.e.l.ly 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 the supporting loops or spires, which serve as one of the special characters distinguishing the _Brachiopods_ from the true Bivalves (_Lamellibranchiata_).]

[Ill.u.s.tration: 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.)]

[Ill.u.s.tration: Fig. 70.--_Pentamerus Knightii_. Wenlock and Ludlow.

The right-hand figure shows the internal part.i.tions of the sh.e.l.l.]

[Ill.u.s.tration: Fig. 71.--Upper Silurian Bivalves. A, _Cardiola interrupta_, Wenlock and Ludlow; B, _Pterinea subfalcata_, Wenlock; C, _Cardiola fibrosa_, Ludlow. (After Salter and M'Coy.)]

[Ill.u.s.tration: 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.)]

[Ill.u.s.tration: Fig 73.--_Tentaculites ornatus_. Upper Silurian of Europe and North America.]

The higher groups of the _Mollusca_ are also largely represented in the Upper Silurian. Apart from some singular types, such as the huge and thick-sh.e.l.led _Megalomi_ of the American Wenlock formation, the Bivalves (_Lamellibranchiata_) present little of 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, 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 sh.e.l.l, 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 sh.e.l.l--often hardly coiled up at all--its minute spire, and its widely-expanded, often sinuated mouth. The British _Acroculioe_ 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 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), _Platyschisma_ (fig. 72, d), _Cyclonema, Pleurotomaria, Murchisonia, Trochonema_, &c. The oceanic Univalves (_Heteropods_) are represented mainly by species of _Bellerophon_; and the Winged Snails, or _Pteropods_, can still boast of the gigantic _Thecoe_ and _Conularioe_, 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 sh.e.l.l in this group is a conical tube, usually adorned with prominent transverse rings, and often with finer transverse or longitudinal striae as well; and many beds of the Upper Silurian exhibit myriads of such tubes scattered promiscuously over their surfaces.

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 _Orthocerat.i.tes_, 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 _Goniat.i.tes_--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.

[Ill.u.s.tration: Fig. 74.--Head-shield of _Pteraspis Banksii_, Ludlow rocks. (After Murchison.)]

[Ill.u.s.tration: Fig. 75.--A, Spine of _Onchus tenuistriatus_; B, s.h.a.green-scales of _Thelodus_. Both from the "bone-bed" of the Upper Ludlow rocks. (After Murchison.)]

Finally, we find ourselves for the first time called upon to consider the remains of undoubted vertebrate animals, in the 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 princ.i.p.al Upper Silurian genus of these is _Pteraspis_, and the annexed ill.u.s.tration (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 p.r.i.c.kly skin or s.h.a.green (_SphaG.o.dus_), along with minute cushion-shaped bodies (_Thelodus_, fig. 75, B), which 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 cla.s.s. 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 cla.s.s 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 ma.s.s of Silurian literature, it is impossible to do more than select a small number of works which have a cla.s.sical 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) 'Ba.s.sin Silurien de Boheme Centrale.' Barrande.

(4) 'Introduction to the Catalogue of British Palaeozoic 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, Winch.e.l.l, Dale Owen, &c.