118 DIMYAIRA. 



21. Large, white, transversely ovate ; vvith broad ribs, provided 

 with vaulted scales ; the posterior slope heart-shaped and gaping. 

 Two feet six inches long, and four feet six inches broad, the 

 largest of all known shells ; sometimes weighing above five 

 hundred pounds weight. Inhabits the Indian ocean. 



ORDER XL— DIMYAIRA. 



Shell having two separate lateral muscular impressions. 

 Section I. — Shell irregular, and always inequivalve. 



TRIBE I. CAMACEA. 



Shell inequivalve, irregular, and adherent ; hinge with one or 

 more teeth ; and with two distinct, lateral, muscular impres- 

 sions. 



Genus 1.— ETHERIA.— ZcmarcA. 



Generic Character. — Shell irregular, inequivalve, adhering; 

 beaks short, and appearing as if sunk in the top of the valves ; 

 hinge destitute of teeth ; waved, somewhat sinuated, and un- 

 equal ; with two distinct, remote oblong muscular impressions ; 

 ligament exterior, tortuous, penetrating partly into the shell. 



The shells of thi? genus diflFer from those of the 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 their base. 



Etheria eUiptica.— The Oval Etheria. Plate XV. fig. 19. 

 Oval, flattened, dilated towards the umbones; the apices re- 

 mote. Inhabits the Indian ocean. 



Genus 2.— CHAMA.— Lamarck. 

 Generic Character. — Inegular, inequivalve, adherent ; beaks 



