GENERA OF SHELLS. 27 



The cytherea is distinguished from the venus by 

 having four cardinal teeth on one valve, and three 

 teeth and the pit on the other. They are all sea shells, 

 solid, in general variously and beautifully colored, free, 

 with the beaks curved and moderately prominent. The 

 oval pit on the left valve corresponds to the msulated 

 tooth of the right. 



I. Internal margin of the valves very entire. 



(a.) Anterior cardinal tooth, icith a striated canal or dentated 



border. 



C. lusoria, petechialis, irnpudica, castanea, zonaria, mere- 



trix, graphica, morplnna, purpurata, casta, corbicula, tripla, — 



convexa. 



(6.) Anterior cardinal tooth loithout striated canal or dentated 



border. 



C. gigantea, erycina, lilacina, impar, erycinella, pecto- 



ralis, planatella, florida, nitidula, chione, maculata, citrina, 



albina, lœta, raactroides, trigonella, sulcalina, hebrsea, cas- 



trensis, ornata, picta, tigrina, venetiana, juvenilis, rufa, guine- 



ensis, didne, arabica, trimaculata, immaculata, pellucida, 



hepatica, lucinalis, lunaris, lactea, exoleta, lincta, concen- 



Hrica, prostrata, interrupta, tigerina, punctata, urnbonella, 



undatina, scripta, nummulina, muscaria, pulicaris, mixta, 



abbreviata, — occuha. 



II. Internal margin of the valves crenated or den- 

 tated. 

 C. pectinata, gibba, ranella, divaricata, testudinalis, cune- 

 ata, placunella, rugifera, plicatina, flexuosa, macrodon, lunu- 

 laris, squamosa, cardilla, cygnus, dentaria. 



Venus. Shell equivalve, inequilateral, trans- 

 verse, or suborbicular. Hinge with three ap- 

 proximate teeth in each valve, the lateral ones 

 diverging at summit ; ligament exterior, cover- 

 ing the labia. 



