cyrexidje. 445 



Fam. CYKENID.F. 



Labial palps lanceolate. Mantle open in front, the 

 margins simple ; siphons short, plain-edged, produced, 

 either partially separated, or completely united to their ex- 

 tremities ; gills two on each side, large, unequal, united 

 behind. Foot large and tongue- shaped. 



Shell more or less tumid, sub-orbicular, closed, covered 

 with a hard, olive, brittle, often polished epidermis ; beaks 

 frequently eroded ; surface of valves concentrically striated 

 or furrowed. Hinge composed of three, or sometimes two 

 diverging cardinal teeth ; lateral teeth compressed ; ligament 

 external. Pallial line simple, or with a slight siphonal 

 inflection. 



Fresh- water ; fluviatile, or estuary. 



Genus CYRENA, Lamarck. 



Mantle-lobes united in their posterior third; siphons 

 separate as far as their bases. 



Shell solid, sub- cordate, ventricose, inequilateral, covered 

 with a horny epidermis ; beaks eroded or decorticated. 

 Hinge with three teeth in each valve, the anterior of right, 

 and hinder of left valve smallest ; lateral teeth smooth, the 

 front roundish, the hinder rather compressed ; ligament 

 external. 



Syn. Cyanocyclas, Ferns. Polymesoda, Rciftn. Cyreno- 

 cjcIsls, Agass. Geloina, Gray. Pseudocyrena, Bourguignat. 



Ex. C. Ceylanica, Chemnitz, pi. 110, fig. 5, 5, a. 



The species of Cyrena inhabit the brackish waters of 

 torrid regions, numerous examples of the genus occurring 



VOL. II. 3 M 



