Fig. 368: Ammonites Bucklandi. Fig. 369: Ammonites planorbis. Fig. 370: Nautilus truncatus. Fig. 371: Ammonites bifrons. 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._ Fig. 372: Ammonites margaritatus. 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. Fig. 373: Extracrinus (Pentacrinus) Briareus. Fig. 374: Palæocoma (Ophioderma) tenuibrachiata. 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. Fig. 375: Scales of Lepidotus gigas. Fig. 376: a. Scales of Æchmodus Leachii, b. Æchmodus (restored outline), c. Scales of Dapedius monilifer. Fishes of the Lias.—The fossil fish, of which there are no less than 117 species known as British, resemble generically those of the Oolite, but differ, according to M. Agassiz, from those of the Cretaceous period. Among them is a species of _ Lepidotus_ (_L. gigas,_ Agassiz), Fig. 375, which is found in the Lias of England, France, and Germany.[2] This genus was before mentioned (p. 316) as occurring in the Wealden, and is supposed to have frequented both rivers and sea-coasts. Another genus of Ganoids (or fish with hard, shining, and enamelled scales), called _Æchmodus_ (Fig. 376), is almost exclusively Liassic. The teeth of a species of _ Acrodus,_ also, are very abundant in the Lias (Fig. 377).