Fig. 1.-Beak and hinge of an INOCERAMUS.

a. The hinge line.

2. Two valves of I. CUVIERI, displaced, and both

showing the external surface.

INOCERAMUS (Lign. 93.).-This name, which refers to the fibrous structure of the shell, has been given to a fossil genus, of which there are about thirty species in the cretaceous and oolitic formations; and very recently four or five species have been discovered in the Silurian strata of Ireland.* These shells are chiefly characterized by their hinge (see Lign. 93, fig. 1 a.), and by the fibrous structure of their constituent substance, which closely resembles that of the recent Pinna; and under the microscope is found, like that shell, to consist of prismatic cells, filled with carbonate of lime. The species vary in size from an inch to three or four feet in diameter. The shell, in consequence of the vertical arrangement of the fibres, readily breaks to pieces, and it is often extremely difficult to extricate a specimen with the hinge and beaks tolerably entire. That they were equally brittle when recent is evident from the numerous fragments diffused through the chalk and flint, and occasionally imbedded in pyrites. The form of

* The term Inoceramus is restricted by the French geologists to the beaked and laminated species of the Galt; and the chalk Inocerami are arranged under the name Catillus.

† Dr. Carpenter on the Microscopical Structure of Shells. To detect this structurc, the shell should be immersed in diluted hydrochloric acid, and, when partially dissolved, the cells will be apparent.

It was many years before I succeeded in obtaining a specimen with the hinge perfect; and M. Brongniart, unable

the hinge is shown in Lign. 93, fig. 1: in the lower specimen two valves of the same individual are seen displaced, one lying over the other. The usual chalk species are figured Foss. South D. Pl. XXVII. and in Min. Conch.

In the Galt, or Folkstone marl, two small species of this genus are to be found in every locality I have visited. They were first figured and described by Mr. Parkinson, under the name of Inoceramus sulcatus, and I. concentricus (Wond. p. 313, figs. 1. and 3.). In most examples the shell is in the state of a white, friable earth, and readily decomposes, leaving patches of iridescent nacre on the casts; but I have seen examples which prove that the originals were of a fibrous structure, like the Inocerami of the Chalk.

The shells of the Inocerami, like those of the oyster, and other living mollusca, were exposed to the attacks of some parasite, or perhaps of some Annelide, as the Nereis. The shells are often quite cellular from this cause, and the cavities are found either hollow, or filled with chalk; or, as in the example Lign. 94, a, with flint. In the latter case, upon the decomposition of the shell, the siliceous casts remain in relief on the surface of the flint, as in Lign. 94, b. Such specimens are common in the

to obtain one from the chalk of France, gave the figure of this genus from my Foss. South D. Pl. XXVII. in the Géog. Min. Env. de Paris.

R

water-worn flints of the South Downs, and in the shingle on the sea-shore of chalk districts; and

Chalk. Lewes.

a. Marks the section of a fragment of shell, with numerous
cavities, occasioned by the depredations of some boring
animals.

b. Portion of shell partially decomposed, and exposing sili-
ceous, globular bodies, connected by filaments, which
are flint casts of hollows occasioned by the ravages of
the borers.

their origin would be difficult to understand without this explanation.*

*The Rev. W. Conybeare first ascertained the origin of these fossils, and figured and described them in an elegant Memoir, published in Geol. Trans. Vol. II. first series.

AVICULA. Ly. II. pp. 59, 93, 109.-Above fifty species of this genus of shells have been found in the British strata; their general character will be readily understood by reference to the pearl-oyster, (Avicula Margaritifera,) which is so largely imported for the manufacture of mother-of-pearl ornaments. A remarkable species is found in the Lias, called, from the great disproportion in the size of the shells, Avicula inequivalvis, Ly. II. p. 59. The recent species are inhabitants of warm climates.

Our limits will not admit of further notice of the Monomyaria, and we proceed to the second division of the plated-gilled mollusca.

DIMYARIA: Bivalve Shells, with two muscular imprints.

The conchifera, or bivalve shells, of this group, fcund fossil, are more than double in number those of the preceding; nearly eight hundred species are known in the rocks of Great Britain, of which by far the greater number is marine. But we must restrict our notice of this division to a few genera, that more space may be devoted to that important class, the Cephalopodous Mollusca.

The Cardium, Venus, and Mussel shells, are familiar examples of the Dimyaria. The conglomerates, now forming in the British Channel, from accumulations of the recent species of Cockle (C. edule)