120 DIMYAIRA. 



beak small, turned to one side and nearly straight ; two separ- 

 ate lateral, muscular impressions ; hinge long, linear, and atten- 

 uated near the centre, tuberculous and nearly crenated ; tubercles 

 unequal ; ligament external and marginal. 



Iridina Nilotica. — The Nile Iridina. Plate XV- fig. 22. 

 Shell transversely oblong; dark olivaceous green on the out- 

 side, somewhat wrinkled concentrically ; inside, fine pearly, 

 with iridescent reflections. Five inches long. Inhabits the 

 Nile. 



Genus 5 ANODONTA Lamarck. 



Generic Gharacter. — Equivalve, inequilateral and transverse ; 

 hinge linear, without teeth ; a glabrous hinge, lamina smooth, 

 truncated or forming a sinus at the anterior end, terminating 

 the apex of the shell j two lateral, remote, twin, muscular im- 

 pressions ; ligament linear, external, sunk in a cleft at the an- 

 terior extremity. 



The substance of the shell is pearly. 



Anodonta cygnea. — The Swan Anodonta. Plate XVI. 

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

 terior side ; with concentric wrinkles ; covered with a green 

 epidermis, frequently brown towards the umbo ; inside pearly 

 white ; beaks small and ventricose. Three inches long, and 

 seven broad. Inhabits fresh water lakes in Europe. 



Genus 6.— HYRIA — Lamarck, 



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

 the apex truncated and parallel, hinge with spreading teeth, 

 the posterior one cleft into numerous divergent portions ; the 

 lateral or anterior tooth strong, elongated and lamellar ; ligament 

 linear, and exterior. 



What distinguishes the shells of this genus particularly is the 

 cardinal or posterior tooth, being divided into many lamellar 

 plaits, or pieces, the centre one being very small. This com- 

 pound tooth is not erect, but slopes in an inclined position to- 

 wards the posterior side. 



Hyria avicularis. — The Little-Bied Hyiua. Plate XVI. 

 fig. 2. Uir.bones smooth and produced •, ears large, with point- 



