[366] The species so abundant at Bognor, is P. brevirostris, Min. Conch. tab. 472. I have seen a block of the limestone, in which, spread over an area of a foot square, there were upwards of fifty specimens lying in relief.
Nucula.—Several species of a small elegant bivalve, related to the preceding, but distinguished by having two rows of teeth on the hinge, diverging from an interspace between the beaks, are found in the Crag and other tertiary deposits (Min. Conch. tab. 180, 192). Two species occur in the Galt (Foss. South D. pl. xix. fig. 5, 6, 9), at Ringmer, Folkstone, Bletchingley, &c., sometimes with the shell perfect, but generally in the state of casts composed of indurated clay, and having impressions of the muscles and of the two rows of hinge-teeth. The shell of one species is marked with fine transverse grooves, or striæ (N. pectinata); the other is of a flattened ovate form, and the surface smooth (N. ovata).
The most beautiful species of Nucula are the N. bivirgata of the Galt of Folkstone, and F. Cobboldiæ of the Norwich Crag.
The species of Nucula with the posterior side produced into a long beak have been separated under the name Leda; they have a pallial sinus, indicating a siphon to the mantle;—
| e.g. | Nucula | ovum | Alum Shale. |
| — | claviformis | Lias. | |
| — | attenuata | Coal Shale. | |
| — | arctica | Norwich Crag. |
PINNA. MYTILUS. MODIOLA.
Pinna.—The common large Pinna, of the Mediterranean, is well known, and differs so entirely from other shells, as to be readily distinguished. There are about fifteen or sixteen British fossil species. The earliest appearance of this genus is in the Carboniferous Limestone of Derbyshire (Phil. York. tab. 6), in which there are two species. The Lias contains one species; the Oolite eight; the Cretaceous formation four; and the London clay two. One of the tertiary species, Pinna affinis (Min. Conch. tab. 313), occurs in considerable numbers in the Bognor rocks, associated with Pectunculi; it varies in length from one to six or seven inches. A beautiful and delicate species is found in the Calcaire grossier of Grignon. Shells of this genus are very rare in the White Chalk, most of the supposed Pinnæ being imperfect examples of Inocerami; but I have seen specimens from Norfolk (collected by the late Mr. Woodward), and one from Sussex, in the cabinet of the Marquess of Northampton.[367]
[367] Dr. Lee has recently discovered in the Kimmeridge Clay on his estate at Hartwell, Bucks, a species of Pinna not previously observed in England. Professor Forbes informs me that it resembles Pinna conica (of Röemer), and is related to P. lanceolata of Sowerby, but appears to be distinct from both.
Mytilus, or Mussel.—There have been found about twenty species of this well-known genus of marine shells in the British strata. They are sparingly distributed through the several formations, from the Silurian to the newer Tertiary. One species (Mytilus Lyellii, Wond. p. 405, fig. 2) occurs in the Wealden, associated with fresh-water shells.
Of the genus termed Modiola, which comprises those mussels that have a rounded anterior termination, nearly forty British species have been discovered; ranging through the fossiliferous strata, from the Silurian to the Crag. A beautiful species (Modiola elegans. Min. Conch. tab. 9), with the shell generally retaining its pearly coat, is found in the London Clay, and in the limestone of Bognor.