DIMYARIA. 



Section I Shell irregular, and always inequi valve. 



TRIBE I. CHAMACEA. 



Shell inequivalve, irregular, and adherent; hinge with one 

 or more teeth ; and two distinct, lateral, muscular im- 

 pressions. 



Genus \.—ETREm A.— Lamarck. 



Generic Character. — Shell irregular, inequivalve, adher- 

 ing; beaks short, and appearing as if sunk in the base of the 

 valves ; hinge destitute of teeth ; waved, somewhat sinuated, 

 and unequal; with two distinct, remote, oblong muscular 

 impressions ; ligament external, tortuous, penetrating partly 

 into the shell. 



The shells of this genus differ from those of Chama, in being pearly 

 in their substance, and lamellar in their texture. 



Section * Shells with an oblong callosity on the base of the valve. 

 ** Shells without an incrusted callosity at theu' base. 



Etheria elliptica. — The Oval Etheria. Plate XV. fig. 

 19. Oval, flattened, dilated towards the luiibones ; the 

 apices remote. Inhabits the Indian ocean. 



Genus 2.— CHAMA Lamarck. 



Generic Character. — Irregular, inequivalve, adherent; 

 beaks incurved, unequal ; hinge 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 lengtlicned callosity, and is usually 

 crenated and grooved. 



The shells of this genus ai-e always affixed to theii- bodies by the 

 lower valves. 



Section * Beaks twisted from left to right. ** Beaks turning fi'ora 

 right to left. 



9l2 



