LAMELLIBRANCHIATA. 75 



prismatic cellular structure of its shell, and the truncation of 

 the posterior (?) side of the valves, which are furnished in 

 some species with a slender siphonal process. 



The Lucinidce, allied to the cockles in their hinge-structure, 

 are also plentiful in the fossil state, and have as wide a range. 

 They are usually recognisable, even when in the condition of 

 internal casts, by their circular form and the oblique ridge on 

 their disk. Casts of Lucina also exhibit the peculiar narrow 

 outline of the anterior adductor detached from the pallial line. 

 Cryptodon, Diplodonta, Kellia, and Pythina are found in the 

 eocene tertiary. Cordis, under the sub-generic form of Spliwra, 

 commences in the trias; another modification, found in the 

 oolites and chalk (Unicardium, d'Orb.), is edentulous; and 

 Tancredia (Lycett), a compressed triangular shell, with a 

 dentition like Cordis, is frequent in the lias and oolite. 



The fresh-water Cycladidce are represented in the Wealden 

 and eocene by many species of Cyrena, mostly of small size. 

 The recent Corhicida fluminalis of eastern rivers is a common 

 fossil of the pliocene tertiary in England and Sicily. 



The Cyprinidce and Astartidce are more abundant as fossil 

 shells, and had a wider range of old than at the present day. 

 Nearly 100 species of Cyprina have been catalogued, com- 

 mencing in the trias; the dentition of the older species is, 

 however, somewhat peculiar. The Isocardice are almost as 

 numerous, and have the same range, but many of the fossil 

 Isocardia-looking shells are really related to the Anatinidce. 

 A yet higher antiquity has been assigned to Cypricardia, a 

 genus now very scarce and difficult to obtain, on account of 

 its habit. The palaeozoic Pleurophorus (fig. 17, 9) is dis- 

 tinguished by the prominent ridge behind the anterior 

 muscular impression ; and Megcdodon (J. Sby.), by the plate 

 supporting the posterior adductor. This genus is represented 

 in the oolites by Pachyrisma (fig. 20, 1), and in the tertiaries 

 and modern seas by Cardilia. 



