502 



LAMELLIBRANCHIATA. 



ventricose ; an external ligament is present ; there is a 

 single cardinal tooth in the right valve, and two small 



Fig. 368. Types of Cyprinidce. A., Interior of left valve of Megalodon cucullatus, showing 

 the adductor sears Devonian ; B, Interior of right valve of Opis lunulata Jurassic ; c, In- 

 terior of right valve of Cardinia Listen Jurassic ; D, Interior of left valve of Phurophorus 

 costatus Permian ; E, Cast of the interior of Cleidophorus planulatus Silurian. (After Wood- 

 ward, King, and M'Coy.) 



teeth in the left, with a single remote lateral tooth in each 

 valve. The genus is doubtfully Silurian, but is represented 

 in all the later Palaeozoic deposits, as well as in the Triassic 

 and Jurassic rocks. 



Another small group of Cyprinidce, allied to the preceding, 

 is represented by Cardita (fig. 369), including cockle-shaped 

 shells, which have radiating ribs, an external ligament, and 

 a toothed margin. The genus commences in the Trias, but 



Fig. 369. Cardita planicosta. Eocene Tertiary. 



attains its maximum in the Tertiary period, about a hundred 

 species having been enumerated from rocks of this age. 



We may, finally, just mention here two or three Palaeo- 

 zoic genera, which would seem to be referable to this family. 

 The first of these is the Lower Silurian Matheria, which has 



