136 DIMYARIA. 



Chama Lazarus. — The Lazarus's Cha.ma. Plate XV. 

 fig. 20. With imbricated, dilated, waved, obsoletely striated 

 foliations; of a white, orange, red, or yellow colour; white 

 within. Two inches in diameter. Inhabits the American 



Genus 3.— DICER AS.— Lamarck. 



Generic Character. — Inequivalve, adherent ; beaks conical, 

 very large, diverging in irregular, spiral contortions ; hinge 

 with a large, thick, concave, somewhat ear-shaped tooth 

 projecting in the larger valve ; two muscular impressions. 



Dicer as arietina. — The Ram-horned Diceras. Plate 

 XVI. fig. 7. Somewhat heart-shaped, with divergent beaks. 

 Fossil. 



Section II. — Lamellipedes. — The foot depressed, 

 lamelliform, and not posterior. 



TRIBE I. :SAYADES. 



Shells inhabiting fresh waters ; the hinge sometimes hav- 

 ing an irregular, simple, or cleft tooth, and a longitudinal, 

 prolonged one, sometimes none; some have irregular, granu- 

 lated tubercles, the whole length of the upper part ; a com- 

 pound posterior muscular impression; umbones often de- 

 corticated. 



Genus 4. — IRIDINA Lamarck. 



Generic Character. — Equi valve, inequilateral, transverse; 

 beaks small, turned to one side and nearly straight ; two 

 separate lateral, muscular impressions ; hinge long, linear, 

 and attenuated near the centre, tuberculous or nearly cre- 

 nated in its whole length ; tubercles unequal ; ligament ex- 

 ternal and marginal. 



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

 22. Shell transversely oblong; dark olivaceous green on 

 the outside, somewhat wrinkled concentrically; inside fine 



