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 { Hard sandstones, conglomerates, { 600 to } Pentamerus lens. Llandovery { and flaggy shaly beds { 1,000 } { Shelly sandstones, shales, and } { Brachiopods; { slaty beds, with grits, con- } { Lamellibranchs; Caradoc or { glomerates, and occasional } 12,000 { Pteropods; Bala { calcareous bands (Bala lime- } { Cystideans; { stone) } { Graptolites; { } { Trilobites. { Dark-grey flagstones, occasionally } Llandeilo { calcareous sandstones, } Flags { with black slates, containing } { Trilobites { Graptolites } 1,000 { (Fig. 36); { } to { Graptolites; } 1,500 { Heteropods; Lower { Dark-grey and ferruginous } { large Llandeilo { slates, sandy shales, and bluish } { Cephalopods. Tremadoc { flags, with occasional beds } Slates { of pisolitic iron-ore }