[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. [Sidenote: 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.