736 



LAMELLIBRANCHIATA. 



is massive, with subspiral beaks and an external ligament. The right 

 valve has two striated cardinal teeth, and the left valve has one, par- 

 tially-divided cardinal tooth. The species of Megalodon range from 

 the Devonian to the Trias, a familiar species being the M. cucul- 

 latus of the Middle Devonian rocks. Allied to the preceding is the 

 Jurassic genus Pachyrisma, in which the shell is thick and pon- 

 derous, and the umbones are subspiral. 



Family 2. Cyprinid^e. — In this family the mantle-lobes are united 

 behind by a curtain pierced by two siphonal orifices, and the foot is 

 thick and tongue-shaped. The shell is equivalve, thick, the beaks 



directed anteriorly and often inrolled, 

 and the hinge is furnished with cardinal 

 and lateral teeth. The ligament is ex- 

 ternal, and the pallial line is entire. 



The principal genus in this family is 

 Cyprina itself, in which the shell is 

 large, strong, and rounded, with a thick 

 epidermis, and a strong external liga- 

 ment. The hinge carries two cardinal 

 teeth in each valve, with a single lateral 

 tooth posteriorly, and variably developed 

 anterior lateral teeth (fig. 616). The 

 genus Cyprina is represented in recent 

 seas by the familiar C. Islandica^ and 

 there are numerous Secondary and Ter- 

 tiary species, commencing in the Lias. 



The genus Isoca?'dia (fig. 617) comprises the "Heart-cockles," 

 in which the shell is cordiform and inflated, and the beaks are 

 remote and subspiral. The ligament splits in front, and is con- 

 tinued in two separate furrows as far 

 as the beaks. The hinge in each valve 

 possesses two cardinal teeth and a pos- 

 terior lateral tooth. The genus Isocardia 

 is represented by living forms, and the fossil 

 species date from the Jurassic rocks. 

 Most of the Jurassic " Heart - cockles," 

 however, belong to the related genus Aniso- 

 cardia, in which the beaks are approximated 

 and the ligament is not split. 



Cypricardia (= Trapezium) has a trapezoi- 

 dal shell, usually radiately striated, the hinge 

 with three radiating cardinal teeth and a pos- 

 terior lateral tooth in each valve, and the posterior side often keeled. 

 The true Cypricardice are probably wholly Mesozoic and Kainozoic, and 

 a few recent species of the genus are known. It is possible, however, 

 that the Palaeozoic genera Cypricardi?iia (Silurian to Carboniferous) and 



Fig. 616. 



Hinge of Cyprina 

 tumida, Pliocene; B, Hinge of Cyp- 

 rina Sanssuri, Cretaceous. 



Fig. 617. — Isocardia crassa. 

 Pliocene. 



