DIMYAIIIA, 119 



incurved, unequal ; hii)ge with one thick, oblique, subcrenated 

 tooth, fitting into a socket in the opposite valve ; two distant 

 lateral muscular impressions ; ligament external, and depressed. 



A strong characteristic mark of these shells is the thick, 

 oblique transverse tooth, which resembles a lengthened callosity, 

 and is usually crenated and grooved. 



The shells of this genus are always affixed to their bodies, by 

 the lower valves. 



Section * Beaks twisted from left to right. ** Beaks turn- 

 ing from right to left. 



Chama Lazarus. — The Lazarus's Cham a. Plate XV. fig. 

 20. With imbricated, dilated, waved foliations, and obsoletely 

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

 within. Two inches in diameter. Inhabits the American seas. 



Genus 3.— DICER AS.— Zawarc/i. 



Generic Character. — Inequivalve, adherent ; beaks conical, 

 very large, diverging in irregular, spiral contortions ; hinge with 

 a large, thick, concave, subauricular tooth in the larger promi- 

 nent valve ; two muscular impressions. 



Diceras ariedna.— The Ram- Horned Diceras. Plate 

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

 Fossil. 



Section 1 1. — Lamellipedes. — The foot depressed, lamelli- 

 form, and not posterior. 



TRIBE I. NAYADES. 



Shells inhabiting fresh waters ; the hinge sometimes having 

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

 one, and sometimes none ; some have irregular, granulated tu- 

 bercles, the whole length of the upper part ; a compound mus 

 cular impression ; and the umbo oftentimes decorticated. 



Genus 4. — IRIDINA.— ZajHorc/^. 

 Generic Character. — Equivalve, inequilatensl, transverse; 



