DIMYAIRA. 129 



short, turgid, and prominent ; the cordiform mark protruding in 

 tlie centre. 



Somewhat allied to the Cyrena, but distinguished by the divergent form 

 of its primary tooth. They all inhabit fresh waters. 



Galathea radiata. — The Rayed Galathea. Plate XVI. 

 fig. 20. Somewhat trigonal, gibbous towards the base ; covered 

 with a yellowish-green thin epidermis, beneath which it is radi- 

 ated with pale chestnut. Three and a half inches long. Inha- 

 bits the rivers of Ceylon. 



Genus 27— CYRENA.— Zamarc/-. 



Generic Character. — Shell roundish and trigonal; turgid and 

 ventricose, inequilateral, solid, with the beaks usually decorti- 

 cated; 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 crenu- 

 lated or serrated. — ** Having the lateral teeth entire. 



Genus 28.— CYCLAS. —Zowiarc^. 



Generic Character. — Ovate, globular, transverse, and equi- 

 vaive, with the beaks tumid ; cardinal teeth very small, some- 

 times barely perceptible ; or with two in each valve, of which one 

 is complicated ; or only one plicated or lobed tooth in one valve, 

 and two in the other ; lateral teeth transversely elongated, com- 

 pressed, and lamelliform ; ligament external. 

 The apices are never eroded in shells of this genus. 

 Cyclas cornea. — The Horny Cylas. Plate XVII. fig. 2. 

 Suborbicular, convex, thin, pellucid, with fine concentric striae ; 

 covered with a horn-coloured epidermis : bluish white within. 

 Three-fourths of an inch long. Inhabits rivers of Europe. 



