Besides the Trilobites, many orders of Mollusca were numerously represented in the Silurian seas. As Sir R. Murchison has observed, no zoological feature in the Upper Silurian rocks is more striking than the great increase and profusion of Cephalopods, many of them of great size, which appear in strata of the age immediately antecedent to the dawn of vertebrated life. Among the Cephalopods we have Gyroceras and Lituites cornu-arietis ([Fig. 21]), whose living representatives are the Nautilus and Cuttlefish of every sea. The genus Bellerophon ([Figs. 54] and [56]), with many others, represented the Gasteropods, and like the living carinaria sailed freely over the sea by means of its fleshy parts. The Gasteropods, with the Lamellibranchs, of which the Oyster is a living type, and the Brachiopods, whose congeners may still be detected in the Terebratula of our Highland lochs and bays, and the Lingulæ of the southern hemisphere, were all then represented. The Lamellibranchiata are without a head, and almost entirely destitute of power of locomotion. Among the Echinodermata we may cite the Hemiscosmites, of which H. pyriformis ([Fig. 22]) may be considered an example.

The rocks of the Lower Silurian age in France are found in Languedoc, in the environs of Neffiez and of Bédarrieux. They occupy, also, great part of Brittany. They occur in Bohemia, also in Spain, Russia, and in the New World. Limestones, sandstones, and schists (slates of Angers) form the chief part of this series. The Cambrian slates are largely represented in Canada and the United States.

Lower Silurian Group.
Formation. Prevailing Rocks. Thickness. Fossils.
Lower Llandovery– Hard sandstones, conglomerates, and flaggy shaly beds –600 to 1,000– Pentamerus lens.
Caradoc or Bala– Shelly sandstones, shales, and slaty beds, with grits, conglomerates, and occasional calcareous bands(Bala limestone) –12,000– Brachiopods; Lamellibranchs; Pteropods; Cystideans; Graptolites; Trilobites.
Llandeilo Flags– Dark-grey flagstones, occasionally calcareous sandstones, with black slates, containing Graptolites –1,000 to 1,500– Trilobites ([Fig. 36]); Graptolites; Heteropods; large Cephalopods.
Lower Llandeilo Tremadoc Slates– Dark-grey and ferruginous slates, sandy shales, and bluish flags, with occasional beds of pisoliticiron-ore
Lingula Flags– Black and dark shaly, grey and brown slaty flagstones and sandstones, with siliceous grits and quartzites –6,000– Trilobites (Olenus, Conocoryphe, Paradoxides, [Fig. 17]); Brachiopods; Cystideans.
Cambrian Group.
Cambrian– Llanberis slates, with sandy strata –3,000– Annelides.
Harlech grits –6,000– Oldhamia.
Laurentian Group.
Upper Laurentian– Stratified, highly-crystalline, and felspathic rocks –12,060– Eozoon.
Lower Laurentian– Gneiss, quartzite, hornblende and mica-schists –18,000– None.

Upper Silurian Period.

Upper Silurian Group.
Lithological Characters. Thickness. Fossils.
Ludlow Rocks– Passage Beds, Tile-stones, and Downton sandstones, at the base of the bone-bed –80– Sea-weeds, Lingulæ, Mollusca.
Micaceous, yellowish and grey, sandy mudstone –700– Crustacea and Fish-remains.
Argillaceous (Aymestry) limestone –50– Crinoids.
Argillaceous Shale with impure limestones –1000– Mollusca of many genera.
Wenlock Rocks– Argillaceous or semi-crystalline limestone –3000– Mollusca of many genera.
Argillaceous shales, in places slatyEchinodermata; Actinozoa; Trilobites.
Woolhope Limestone and occasional bands of argillaceous nodulesGraptolites.
Upper Llandovery Rocks– Grey and yellowish sandstones (occasionally conglomerates) with bands of limestone –800– Pentamerus oblongus, Rhynchonella, Orthides, &c.

Among the fossils of this period may be remarked a number of Trilobites, which then attained their greatest development. Among others, Calymene Blumenbachii ([Fig. 23]), some Cephalopoda, and Brachiopoda, among which last may be named Pentamerus Knightii, Orthis, &c., and some Corals, as Halysites catenularius ([Fig. 26]), or the chain coral.

Fig. 23.—Calymene Blumenbachii partially rolled up.