PELECYPODS 453 



There is another family of rock- and mud-boring bivalves which 

 superficially resemble the petricolas, but their anatomical organization 

 removes them to a little distance from the Petricolidee. They are COP 

 sidered further on, under the name of Pholadidte. 



FAMILY CABDIIDJE 

 GENUS Cardium 



This is another important family of the Pelecypoda, which, 

 with the Veneridce, the Mactridw, and the Tettinidce, is entitled to 

 distinction on account of its high organization, its high type of 

 shell, its universal geographical distribution, and its great beauty. 

 The dominant genus is Cardium a, name given on account of the 

 fact that the shell is shaped like a heart. The animal has a very long, 

 recurved, and surprisingly strong foot. The mantle-edges are quite 

 freely open to the siphonal region and are highly papillate and 

 waved. The siphons are usually very short, but have wide exterior 

 openings and are also strongly papillate about their margins. 

 The gills are obliquely situated, and seem to be thicker and heavier 

 than is usual in pelecypods ; they are much folded, the outer 

 lamella being the smaller. With such a pointed foot and such 

 short siphons Cardium is obviously not a deep-burrowing genus ; 

 but it does partially bury itself in soft semi-liquid sand. Its pro- ' 

 gression is effected by leaps which it is enabled to make by reach- 

 ing far out with its foot, then suddenly twisting to one side and 

 throwing the shell several inches. The shell is ventricose, is 

 closed or gapes slightly posteriorly, and has prominent subcentral 

 umbones and an external ligament. The sculpture-scheme of 

 Cardium (if the shell is not perfectly smooth) is one of radiating 

 ribs or ridges, which, being regular, form a perfect crenulation of 

 the ventral margin. There are two cardinal teeth in the center, 

 and one lateral tooth upon both the anterior and the posterior side : 

 the latter are situated at some little distance from the beaks. 



C. magnum. The largest and finest ' ' cockle " of the east coast of the 

 United States. It is, indeed, one of the finest cockles in the world. The 

 almost perfect heart-shape is striking. The posterior side is some- 

 what flatly depressed. The thirty-three to thirty-seven regularly dis- 

 posed, broad, radiating ribs ; the regularly crenulated margins ; the 

 yellowish-brown color garnished with transverse rows of chestnut or 



