HALACOZOA. TROPIOPODA. LAMELLIBRANCHIATA. 271 



and two deep pits in eacli valve, in the riglit two distant 

 elongated lateral teeth and two submarginal grooves, 

 two prominent lateral teeth in the left valve, with two 

 pits for the teeth of the other. Ligament short, exter- 

 nal, prominent. Muscular impressions large. 



They live in sand, gravel, or mud, in shallow or deep 

 water. By the action of the large foot, they can shift 

 their position, and even leap to a considerable distance, 

 but it does not appear that they habitually move thus 

 from place to place. Their position is with the broader 

 or anterior end downward. 



1. Cdrdiuin echindtiun. Prickly Coclde. 



Shell cordate, obliquely roundish, subequilateral, tumid ; 

 with the dorsal margin short, but forming an angle behind ; 

 the umbones very prominent, curved a little forwards ; each 

 valve with twenty rather tliick, convex ribs, which have 

 a medial thin lamina, serrated for some way from the um- 

 bones, but from about the middle of their length presenting 

 numerous spines, which, on the anterior side of the shell, are 

 curved backwards, somewhat spathulate, but acute, on the 

 other parts straight and acute ; the interstices between the 

 ribs transversely rugous ; the colour whitish, often blotched or 

 banded with reddish ; the inner surface white, with the ribs 

 apparent, and the margins serrato-plicate ; two approximated 

 tapering teeth in each valve ; the lateral teeth prominent. 

 Length two inches and a-half, breadth two inches and eight- 

 twelfths. 



It varies considerably. Sometimes, the form being the 

 same, the valves are much thicker, with the same number of 

 ribs, which are much stronger, with a medial, little elevated 

 lamina, or even grooved, and only marked toward the margin 

 with opines or tiwercles, which are induplicatc, tubular, or 

 cochleariform, fewer than in the variety above described, and 

 not so prominent. This variety attains a diameter of four 

 inches, and is the Cardium aculeatum of authors. 



Young shells are very thin, transparent, white, with the 

 medial lamince of the ribs forming compressed eminences. 

 They are the Cardium ciliare of some authors. 



Not very uncommon off the sandy coast. Tb.e individuals 

 found in sand, and near the shore, are of the second variety j 

 those from deep water always of the first. 



