474 CYPRlNIDiE. 



increase, which as usual become more marked as age ad- 

 vances. The ventral margin is arcuated, and rises sharply 

 in front ; the hinder dorsal edge is convex, and slopes so 

 little; (indeed it slightly ascends at first) as almost to Le 

 sub-parallel with the opposite margin. The front dorsal 

 slope is short, incurved, and rather abrujit. The anterior 

 extremity is angulated, the angle being, in the adult, above 

 the middle of the side, and its tip rounded oif ; the pos- 

 terior termination is rather broad, and very bluntly bian- 

 gulated. The umbones are contorted, and project consi- 

 derably forward, so as to appear subterminal ; the beaks 

 are so much inflected as almost to appear spiral, and so 

 diverge from each other as to be widely sundered at their 

 apices, which are very acute, and lean upward. The liga- 

 ment, which, though sufficiently ample, is not particidarly 

 prominent, is divided anteriorly, and runs in a narrow slip 

 along the base of each umbo. There is no defined lunule, 

 but a very expansive hollow in front of the umbones ; the 

 dorsal lips, however, are elevated. Within, the surface is 

 of a whitish hue, with occasionally a slight tinge of buff or 

 flesh-colour upon its disk ; the inner edge is sharp and 

 simple. The teeth of the hinge are erect, and lie parallel 

 with the margin ; in the left valve the front one is very 

 large, truncato-trigonal, indented at its apex so as at times 

 almost to appear double, and impressed below by the op- 

 posing tooth ; the hinder one is thin, laminar, and dorsal. 

 Of the two strong front primary teeth of the right valve, 

 which lie parallel with each other, the upper is semicircular 

 and situated less forward, the lower one is the shorter, and 

 more trigonal ; the hinder tooth is laminar, and not pecu- 

 liarly solid. The lateral tooth (and its tooth-like recep- 

 tacle) are strong, posterior, and remote fi-om the beaks, 

 lying beyond the termination of the ligament. 



