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 Petricolide. They are con- 
sidered further on, under the name of Pholadide. 
FAMILY CARDIDE 
Genus Cardium 
This is another important family of the Pelecypoda, which, 
with the Veneride, the Mactride, and the Tellinide, 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 Cardiwm—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 Cardiwm 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 
