CONCHIFERA OK BIVALVES. 95 



54. Cucull^ea. — Shell equivalve, inequi- 

 lateral; bosses not touching each other; muscu- 

 lar impression forming ^^S^r^x 



small teeth; ligament ^< v :.^.^^ 



external. Animal, foot large. — 1 species recent, 

 6 fossil. 



This shell greatly resembles the Area : there 

 is but one species, which is found in the sand 

 in the Indian Ocean : its colour is a deep cin- 

 namon brown externally, and brown tinged 

 with violet internally. 



> 55. Arca. — Shell transverse, nearly equi- 

 valve, inequilateral, heart-shaped, valves ribbed, 



and in some species gaping at the lower part ; 

 hinge straight ; bosses prominent and distant, 

 and often a little recurved ; ligament external. 



■ partly detached shelly /SSI 

 plate within; hinge / J/ 

 straight, with a row of V \V 



