16 CARDIACS. 



monness of tlie shells, which offers every facility for ascer- 

 taining the exact extent of the species, the latter state has 

 been separated by Lamarck, and some of his followers, as a 

 distinct species, under the name of G. crenulatum. 



The shape of the adult Cockle is subovate and subcordate, 

 ranging occasionally to subtrigonal and suborbicular ; in 

 the latter case, the valves, which are always ventricose 

 and typically inequilateral, become eminently swollen, and 

 nearly equilateral. The texture is opaque, solid (occa- 

 sionally ponderous), and of a squalid white, frequently with 

 a ferruginous cast, but never marbled nor variegated exter- 

 nally. 



The outer surface is dull, and occasionally covered in 

 part with an ashy-olive fugacious epidermis, usually con- 

 fined, however, to the posterior end, and the vicinity of the 

 lower margin. About twenty to twenty-six radiating ribs 

 are visible, which are tolerably evenly diffused over the 

 entire surface ; the two or three which cover the ordinary 

 site of an umbonal ridge are, however, rather the largest, 

 and the succeeding ones decidedly the narrowest of the 

 series. These ribs are but little elevated (the terminal ones 

 are, indeed, greatly depressed), and only separated from 

 each other, in the more characteristic examples, by narrow 

 grooves on the central disk, and concave (not square cut), 

 rather broader ones at the sides. Crowded and slightly- 

 curved linear elevations concentrically traverse the summit 

 of the ribs, but become obsolete upon the posterior ones. 

 The ventral edge, which is always more or less curved, and 

 occasionally much arcuated, usually displays less convexity 

 behind than in front, where it ascends with a bold sweep, 

 forming a well-rounded and not very broad anterior ex- 

 tremity. The dorsal edges are straightish, or even subre- 

 tuse, and decline but in a trifling degree ; the posterior edge 



