Plicatula, is another genus of this family, of which there are three British fossil species. A delicate shell, with slender depressed spines (P. inflata. Foss. South D. pl. xxvi.), occurs in the Chalk Marl. The recent species are natives of the seas of warm climates.
Pecten.—The common scallop-shell will serve as a type of this genus. The animals of these shells, unlike the oysters, have the power of locomotion, and when in the water, may be seen moving with rapidity, and flapping their shells to and fro with great activity. Numerous species are found fossil. In the Pliocene, and other marine tertiary deposits, Pectens abound; in the White Chalk there are several elegant forms (see Foss. South D. plate xxv.); many kinds in the Oolite and Lias; and several in the Devonian strata.
A large Mediterranean species (Pecten Jacobæus, Ly. p. 152) occurs in the Pliocene strata of Palermo, in every stage of growth, and as perfect as if recent. The Chalk and Shanklin sand contain a small inequivalved Pecten, the lower valve of which is convex, and pentangular, the upper flat, and both strongly ribbed, or pectinated; it is named Pecten quinquecostatus (Foss. South D. pl. xxvi. Ly. p. 212); and in the cretaceous strata of North America a variety of this species is found.
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).
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. |
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.