DIMYARIA. 147 



Genus 27— CYRENA Lamarck. 



Generic Character. — Shell roundish or trigonal; turgid 

 and ventricose, inequilateral, solid, with the beaks usually 

 decorticated ; hinge with three teeth in each valve ; lateral 

 teeth two, one of which is near the primary ones ; ligament 

 external ; a great part of which is inserted, and placed on 

 the largest side. 



The apices are always eroded or carious in shells of this genus. 

 They frequently grow to a large size, and always inhabit rivers. 



Cyrena fluminea. — The River Cyrena. Plate XVII. 

 fig. 1. Heart-shaped, gibbous, greenish-brown ; transversely 

 sulcated; variegated with white and violet in the inside. 

 One inch long. Inhabits the rivers of China. 



This genus has two sections. — * With the lateral teeth crenulated or 

 sen-ated. — *• Having the lateral teeth entire. 



Genus 28— CYCLAS — Lamarck. 



Generic Character. — Ovate, globular, transverse, and 

 eqiiivalve, with the beaks tumid; cardinal teeth very small, 

 sometimes barely perceptible ; or with two in each valve, oj 

 which one is complicated ; or only one plicated or lobed tooth 

 in one valve, and two in the other; lateral teeth trans- 

 versely elongated, compressed, and lamdJiforra ; ligament 

 external. 



The apices are seldom eroded in shells of this genus, of which all the 

 species inhabit fi-esh-vvater. 



Cyclas cornea. — The Horny Cyclas. Plate XVII. 

 fig. 2. Suborbicular, convex, thin, pellucid, with fine con- 

 centric striae ; covered with a horn-coloured epidermis ; 

 bluish white within. Three-fourths of an inch long. In- 

 habits rivers of Europe. 



Genus 29— PISIDIUM.— P/ei/fer. 



Generic Character. — Shell equivalve, transverse, wth 

 the sides unequal, completely closing; in the right valve 

 n2 



