40 GENERA OF SHELLS. 



irregular spires ; tooth very large, thick, con- 

 cave, sub-auricular, prominent in the largest 

 valve ; two muscular impressions. 



The diceras somewhat resembles isocardium, but is 

 more nearly allied to chama. It differs from them by 

 its hinge, and the singular form of the beaks. 



Fossil. Fifty three species enumerated. 



Chama. Shell irregular, inequivalve, adhe- 

 rent ; beaks incurved, unequal. Hinge with a 

 single thick tooth, oblique, subcrenate, inserted 

 into the fossa of the opposite valve ; two dis- 

 tant, lateral muscular impressions ; ligament 

 external, depressed. 



This genus now consists of irregular, coarse, rough, 

 scaly, or spinous shells, with very unequal valves, and 

 only one thick, oblique, transverse, callous tooth, usu- 

 ally crenate or furrowed. The beaks are curved in- 

 wards, and only one of them projects at the base of the 

 shell. They live in shallow water, attached by the lar- 

 ger valve to rocks, corals, or to each other. They are 

 seldom brilliantly colored. 



(a.) Beaks turning from left to right. 



C. lazarus, damaecoriiis, gryphoides, crenulata, unicornis ; 

 florida, limbulu, œruginosa, asperella, decussata. 

 (h) Beaks turning from right to left. 



C. arcinella, radians, cristella, albida, ruderalis, croceata, 

 Japonica. 



Etheria. Shell irregular, inequivalve, adhe- 

 rent ; beaks short, as it were sunk in the base 

 of the valves. Hinge toothless, undate, sub- 

 sinuate, unequal ; two distant, oblong, lateral 



