In the Chalk Marl a large and beautiful Pecten (_P. Beaveri. Min. Conch._ tab. 158) is very common, and I have obtained from Hamsey and Southerham examples in the most perfect state of preservation; it is a characteristic shell of the Chalk Marl of England (_Foss. South D._ plate XXV. fig. 11). [Illustration: Lign. 129 Inoceramus Cuvieri. _Chalk. Lewes._ 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. ] [Sidenote: INOCERAMUS.] Inoceramus. _Lign. 129._--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.[361] These shells are chiefly characterized by their hinge (see _Lign. 129, fig. 1a._), and by the fibrous structure of their constituent substance, which closely resembles that of the recent _Pinna_;[362] and under the microscope is found, like that shell, to consist of prismatic cells, filled with carbonate of lime.[363] 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.[364] The form of the hinge is shown in _Lign. 129, 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._ [361] 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_. [362] _Perna_ and all the Aviculidæ have the same structure, Inoceramus scarcely differs from Perna. [363] Dr. Carpenter on the Microscopical Structure of Shells. To detect this structure, the shell should be immersed in diluted hydrochloric acid, and when partially dissolved, the cells will be apparent. [364] It was many years before I succeeded in obtaining a specimen with the hinge perfect; and M. Brongniart, unable 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.