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.
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.
Allusion has already been made, p. 354, to numerous zones in the Lias having each their peculiar Ammonites. Two of these occur near the base of the Lower Lias, having a united thickness, varying from 40 to 80 feet. The upper of these is characterised by Ammonites Bucklandi, and the lower by Ammonites planorbis (see Figs. 368, 369).[[1]] Sometimes, however, there is a third intermediate zone, that of Ammonites angulatus, which is the equivalent of the zone called the infra-lias on the Continent, the species of which are for the most part common to the superior group marked by Ammonites Bucklandi.
Among the Crinoids or Stone-lilies of the Lias, the Pentacrinites are conspicuous. (See Fig. 373.) Of Palæocoma (Ophioderma) Egertoni (Fig. 374), referable to the Ophiuridæ of Muller, perfect specimens have been met with in the Middle Lias beds of Dorset and Yorkshire.
The Extracrinus Briareus (removed by Major Austin from Pentacrinus on account of generic differences) occurs in tangled masses, forming thin beds of considerable extent, in the Lower Lias of Dorset, Gloucestershire, and Yorkshire. The remains are often highly charged with pyrites. This Crinoid, with its innumerable tentacular arms, appears to have been frequently attached to the driftwood of the liassic sea, in the same manner as Barnacles float about on wood at the present day. There is another species of Extracrinus and several of Pentacrinus in the Lias; and the latter genus is found in nearly all the formations from the Lias to the London Clay inclusive. It is represented in the present seas by the delicate and rare Pentacrinus caput-medusæ of the Antilles, which, with Comatula, is one of the few surviving members of the ancient family of the Crinoids, represented by so many extinct genera in the older formations.