CONCHIFERA. 



231 



scarcely a sinus behind. Animal, with an open mantle, 

 long cirrhi behind, and two siphons. — 1 species. 



A solitary imperfect specimen has been brought from 

 the North Sea. 



(Fluviatile genera, with the ligament external.) 



GrALATEA. Bnig. (Egeria Roissy ; Megadesma Bow- 

 dich; Potamophila Sow.) — Shell equivalve, thick, 

 solid, and closed, somewhat triangular, nearly equilateral, 

 epidermis greenish and horny ; cardinal 

 teeth two in one valve, and three in 

 the other, lateral teeth very indis- 

 tinct ; ligament external, projecting 

 and swollen; the pallial impression 

 has a sinus behind. Animal with ge- 

 nerally an open mantle, the hinder 

 part running out into two straight, divided siphons ; the 

 foot is large, rather long, compressed ; on each side are 

 two unequal gills, the outer of which is joined to the 

 middle of the upper edge of the inner gill ; which is an 

 unusual formation; the mouth is large, the four mouth 

 tentacles triangular. — 6 or 7 species. 



The species are found in the rivers of Africa, and are 

 rare shells ; G. radiata the most common. 



Ctrena. Lam. — Shell thick, inequilateral, and round 

 in form ; cardinal teeth three in each valve, 

 lateral generally one ; ligament external, 

 and on the longest side ; bosses eroded ; an 

 epidermis. Animal very similar to the 

 Cyclas. — 20 species recent; also fossil. 



Natives of rivers in warm climates, par- 

 ticularly in Asia : the shell has the interior 

 of a beautiful violet colour, the outside being c y rena & 

 occasionally tinged with the same. 



