DIMYARIA. 135 



Section I — Shell irregular, and always inequivalve. 



TRIBE I. CHAMACEA. 



Shell inequivalve, irregular, and adherent; hinge with one 

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

 pressions. 



Genus l.—ETRERI A.— Lamarck. 



Generic Character. — Shell irregular, ineqiuvalve, adher- 

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

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

 and imequai ; 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 textm-e. 



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



** Shells without an incrusted callosity at theii- base. 



Etheria elliptica The Oval Etheria. Plate XV. fig. 



19. Oval, flattened, dilated towards the umbones ; 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 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 turning from 

 right to left. 



m2 



