In the whole of the English Lias there are at present known about 937 species of mollusca, and of these 267 are Cephalopods, of which class more than two-thirds are Ammonites, the Nautilus and Belemnite also abounding. The whole series has been divided by zones characterised by particular Ammonites; for while other families of shells pass from one division to another in numbers varying from about 20 to 50 per cent, these cephalopods are almost always limited to single zones, as Quenstedt and Oppel have shown for Germany, and Dr. Wright and others for England. As no actual unconformity is known from the top of the Upper to the bottom of the Lower Lias, and as there is a marked uniformity in the mineral character of almost all the strata, it is somewhat difficult to account even for such partial breaks as have been alluded to in the succession of species, if we reject the hypothesis that the old species were in each case destroyed at the close of the deposition of the rocks containing them, and replaced by the creation of new forms when the succeeding formation began. I agree with Professor Ramsay in not accepting this hypothesis. No doubt some of the old species occasionally died out, and left no representatives in Europe or elsewhere; others were locally exterminated in the struggle for life by species which invaded their ancient domain, or by varieties better fitted for a new state of things. Pauses also of vast duration may have occurred in the deposition of strata, allowing time for the modification of organic life throughout the globe, slowly brought about by variation accompanied by extinction of the original forms. Fig. 361: Plagiostoma (Lima) giganteum. Fig. 362: Gryphæa incurva. Fossils of the Lias.—The name of Gryphite limestone has sometimes been applied to the Lias, in consequence of the great number of shells which it contains of a species of oyster, or _ Gryphæa_ (Fig. 362). A large heavy shell called _Hippopodium_ (Fig. 365), allied to _Cypricardia,_ is also characteristic of the upper part of the Lower Lias. In this formation occur also the Aviculas, Figs. 363 and 364. The Lias formation is also remarkable for being the newest of the secondary rocks in which brachiopoda of the genera _Spirifer_ and _ Leptæna_ (Figs. 366, 367) occur, although the former is slightly modified in structure so as to constitute the subgenus Spiriferina, Davidson, and the Leptæna has dwindled to a shell smaller in size than a pea. No less than eight or nine species of Spiriferina are enumerated by Mr. Davidson as belonging to the Lias. Palliobranchiate mollusca predominate greatly in strata older than the Trias; but, so far as we yet know, they did not survive the Liassic epoch. Fig. 363: Avicula inæquivalvis. Fig. 364: Avicula cygnipes. Fig. 365: Hippopodium ponderosum. Fig. 366: Spiriferina (Spirifera). Fig. 367: Leptæna Moorei.