50 CONCHOLOGY. 



10. Genus Crassina. 

 Animal. As above. 



Shell. Distinguished from the Venus by having only tv^ro teeth on 

 each valve, and from the crasatella by the position of the ligament ; 

 solid, sub-orbicular, thick, hinge with tvro strong diverging pri- 

 mary teeth in the right valve, and tvv^o unequal ones on the other ; 

 ligament external. Inhabits the Scottish and Devonshire coasts. 

 One species. 



Crassina Danmoniensis. 



FAMILY IX. 



CoNCHACEA. Seven Genera. 



This family is divided into Conchse marins!, and Conchsf. 

 fiuviatiles. 



CoxcH.E Marinje. Four Genera. 



1. Cyprina. 



Animal. Thick, oval, foot compressed, falciform, geniculated ; 

 mantle closed behind, and pierced by two oval apertures with 

 cirrous edges ; no veritable tubes. 



Shell With epidermis ; thick, regular, substriated longitudi- 

 nally, subcordiform,^ equivalved, inequilateral ; summits strongly 

 flexed to the front and often contiguous; hinge thick, subsimilar, 

 formed by three slightly converging cardinal teeth, and by a pos- 

 terior lateral tooth, sometimes obsolete ; ligament very thick ; mus- 

 cular impressions distant, subcircular, and united by a narrow mar- 

 ginal band. Inhabits the Atlantic ocean and British seas. Two 

 living species. Seven fossil. 



Cyprina tennistria. Cyprina Icelandica» 



3. Genus Cytherea. 



Animal. Oval or round, generally but little compressed ; edges 



of the mantle undulous, and garnished with tentacular cirrhi in 



one row; foot considerable, compressed, trenchant, in other 



respects diversiform ; tubes tolerably elongated, and most usually 



