CONCHOLOGY. 69 



thicker and more solid than the Anodonta, and of a brilliant 

 rose-coloured pearly hue ; it is found in the rivers of warm 

 countries. 



Shell equivalve, inequilateral, transverse, with small apices; 

 recurved, but nearly erect ; not auriculated ; hinge very long, 

 linear, crenulated through all its length; ligament external 

 and marginal ; two well-marked muscular impressions. 



/. exotica. The exotic Iridina. 



The only species of this genus answering to the above 

 description. 



FAMILY XIV. 



Chamacea. Three genera. 

 1. Diceras. One fossil species. 



Only found in a fossil state ; distinguished from the Chama 

 by the hinge, which is dissimilar, formed by a large thick 

 tooth, concave in the greater valve ; summits very project- 

 ing ; almost regular spiral contortions. 



D. arietina. The ram's-horn Diceras. 

 Shell irregular, inequivalve, inequilateral, somewhat heart- 

 shaped, with divergent beaks. 



2. Chama. The Clam or Gaper. Seventeen species. 



In this genus are now comprehended only such as have a 

 thick oblique transverse tooth, resembling a lengthened cal- 

 losity, generally crenulated or grooved, fitting into a corre- 

 sponding cavity in the lower valve. The animals inhabiting 

 these shells have the faculty of affixing themselves to other 

 bodies by the lower valve. 



The Chama received its name from its gaping ; it is found 

 in most seas, particularly in the Southern ; sometimes its 

 colours are elegantly blended. 



Shell irregular, adherent, inequivalve, inequilateral ; sum- 

 mits more or less twisted spirally, especially in the lower 

 valve; some from left to right, others from right to left; 



