32 GENERA OF SHELLS. 



ARCACEA. 



Cardinal teeth small, numerous, entering, disposed in each 

 valve in a straight, arched or broken line. 

 The arcacea are very remarkable by the hinge of 

 their shells, which are equivalve, regular, the beaks 

 commonly distant, the ligament entirely external, and 

 the muscular impressions lateral. Several have a vel- 

 vety epidermis ; some adhere to rocks by tendinous 

 threads, and the shell gapes more or less at the supe- 

 rior margin. They are all sea shells, and live near the 

 shore. 



CucuLLiEA. Shell equivalve, inequilateral, 

 trapeziform, ventricose ; beaks distant, separa- 

 ted by the facet of the ligament ; anterior 

 muscular impression, with an angular or auri- 

 cular margin. Hinge linear, straight, with 

 minute transverse teeth ; having at each ex- 

 tremity, two to five ribs parallel to it ; liga- 

 ment entirely exterior. 



The cucullœa are large, very tumid shells, with the 

 anterior side obliquely truncated, forming a broad, cor- 

 date, flattened corselet, slightly elevated towards the 

 middle. The hinge is that of the area, but as the shell 

 grows old, it is displaced, and leaving the remains of 

 its former margins at the extremities, gives rise to two 

 parallel ribs by which it is terminated, which is not 

 seen in the area. 



C. auriculifera, crassatina. 



Arca. Shell transverse, subequivalve, ine- 

 quilateral, beaks distant, separated by the area 



