DIMYARIA. 137 



pearly, with iridescent reflections. Five inches long. In- 

 habits the Nile. 



Genus 5.— ANODONTA.— Za/?mrc/^. 



Generic Character. — Equivalve, inequilateral, transverse; 

 hinge linear, without teeth ; an adnate, smooth, cardinal 

 lamina, truncated or forming a sinus at the anterior end, 

 terminating the base of the shell; two lateral, remote, twin, 

 muscular impressions; ligament linear, external, sunk in a 

 cleft at the anterior extremity. 

 The substance of the shell is pearly. 



Anadonta cygnea. The Swan Anadonta. Plate XVI. 



fig. 1. Ovate, thin, convex, somewhat compressed on the 

 anterior side ; with concentric wi'inkles ; covered w\i\\ a 

 green epidermis, frequently brown towards the umbo; in- 

 side pearly white ; beaks small and ventricose. Three 

 inches long, and seven broad. Inhabits the fresh- water lakes 

 in Europe. 



Genus 6. — HYRIA. — Lamarck. 



Generic Character. — Equivalve, oblique, trigonal, eared, 

 "with the base truncated and straight, hinge with two spread- 

 ing teeth, the posterior one cleft into numerous divergent 

 portions; the lateral or anterior tooth strong, elongated, and 

 lamellar ; ligament linear, and external. 



What distinguishes tlie shells of this genus particulai'ly is the cai*- 

 dinal or posterior tooth being divided into many lamellar plaits, or 

 pieces, the centre one being very small. This compound tooth is not 

 erect, but slopes in an inclined position towards the posterior side. 



Hyria avicularis. — The Little-bird Hyria. Plate 

 XVI. fig. 2. Umbones smooth and produced, in the form 

 of a tail ; ears large, with pointed terminations ; a greenish- 

 brown epidermis, and finely striated ; inside pearly, and of a 

 rich reddish golden-yellow. Three inches broad. Inhabits 

 the rivers of America. 



