are conspicuously abundant; masses of the most beautiful limestone having the stalks and calices of these crinoids scattered throughout the rock in profusion. Of the crustacea, the trilobites still remain examples, but there is now added to them a gigantic crustacean with very distinct swimming-paddles, and arms bearing at their extremities claws similar to those of a prawn. This formidable creature, Pterygotus, was frequently six

FIG. 10.-Upper Silurian Trilobites.

a. Cheirurus bimucronatus; b. Phacops longicaudatus; c. Phacops Downingiæ; d. Harpes Ungula. feet in length, and must have swept the Upper Silurian seas undisputed by any rival, except, possibly, the fishes, which are represented only as yet by scales and curious head-shields, and are believed to belong to the ganoid group of the sturgeons and gar-pikes. If these are remains of fishes, then we have the first appearance of

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any vertebrate, and that of the lowest type, though not until all the inferior classes of marine animals had subsisted for a prolonged period.

Reference has been made to the oil-bearing shales, and the theory that their hydro-carbons are distilled from the countless numbers of Silurian trilobites. In any case, this was the culminating period of their existence, distinctively the "Age of Trilobites;" and the museums contain specimens of rock almost wholly composed of densely impacted trilobites. The rocks of the system are widely distributed, and have yielded oil in Europe, Asia, and America abundantly. They stretch through Wales, Cumberland, Westmoreland, the south of Scotland, Ireland, and France; through Scandinavia, Russia, Bohemia, Asia Minor, Northern India, Australia, New Zealand, and both Americas; yielding gold, silver, platinum, mercury, tin, manganese, and other metals or ores. Though the life of the period was distinctively marine, and widely extended oceans prevailed, large continents must necessarily have existed to provide material for the accumulation of these rocks. Palæontologists do not even despair of eventually finding evidence of considerable vegetation, and of the presence of true fishes; although there is no probability that anything will be found to invalidate the broad principle of the succession of life. Since it is believed that we are already acquainted with the predominant forms, it is scarcely likely that any additions to the list will include organisms of more advanced type than a lepidodendron or a fish.

CHAPTER IX.

THE DEVONIAN, CARBONIFEROUS, AND PERMIAN

LIFE-PERIODS.

Distinction between "Devonian" and "Old Red"-Coal seams— -Vegetation of the period-Placoid and ganoid fishes-Carboniferous vegetation, mollusca, crustacea, fishes, amphibia, and insects-The Permian period-Disappearance of sigillaria-First appearance of true reptiles-Proterosaurus.

BELOW the Carboniferous is an extensive and important series of rocks, formerly known under the name of Old Red Sandstone; in contradistinction to the sandstones, etc., immediately above the Carboniferous-now forming the Permian and Triassic groups, but originally classed as New Red Sandstone. Whether we elect to accept the term Devonian as inclusive of all the rocks belonging to the sub-Carboniferous series, or retain the name "Old Red" for a portion of them, the distinction is of little importance in relation to their age. They belong in all probability to the same life-period, though this distinction may be drawn between the Devonian and Old Red: the former seem to represent deep-sea conditions in America, Europe, and Britain; while the latter are probably shoal-water deposits-possibly, in some cases,

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of inland seas-in Canada and Scotland. Practically, they may be regarded as contemporaneous, representing different geographical areas and dissimilar local influences. The great majority of geologists accept this view, advanced by Murchison and Sedgwick, and it is likely to be retained until a much fuller acquaintance with the distribution of the rocks and fossils of the system may afford ground for modifying it.

Devonian rocks, it should be observed, occur in large districts in South America and Australia, as well as in Europe and North America. Here for the first time we find coal-seams, indicative of an abundance of terrestrial vegetable life, which is wanting or at least as yet unknown-in any former period. Most of the examples of fossil plants have been obtained from the Gaspé rocks of the Old Red division—a fact which helps to fix the character of these sandstones as shallow-water deposits; and where they pass upwards into the Carboniferous system, there is little distinction between the flora of the two periods. Ferns of several species, tree ferns of large size-lepidodendra, sigillaria, calamites--a singular plant named by Principal Dawson Psilophyton, from the Devonian of Canada, and a tree trunk a foot and a half in diameter (Prototaxites), besides the exogenous forms Dadoxylon and Ormoxylon, afford conclusive evidence of the prevalence of extensive tracts of land richly clothed with vegetation.

Not less remarkable is the advance in marine life. True sponges, corals of various forms, and, among shellfish, Spirifera, Atrypa, Strophomena, Spirorbis (a tube

inhabiting sea-worm), and the chambered shells Clymenia and Goniatites are prominent objects. Of the trilobites, most of the genera are survivals, but in Phacops latifrons we have an apparently new and widely distributed form; and the small Cypridine, whose fossil bodies constitute masses of slaty rock, further exemplify crustaceous life. The graptolites, as a group, have now disappeared, and are never revived, although one insignificant inhabitant of existing seas (Dictyonema) may be a faint representative of this great Silurian family.

But Devonian zoology is richest in marine vertebrates -fishes of the two great placoid and ganoid groups,

FIG. 11.-A. Polypterus, a recent ganoid fish; B. Ostolepis, a Devonian ganoid fish; a. a. pectoral fins, with rays surrounding a central lobe b. c. ventral fins; d. d'. dorsal fins, with a spine in front.

represented respectively by sharks, dog-fishes, skates, and rays; and by sturgeons and gar-pikes. Both groups are distinguished by an almost wholly cartilaginous