CONCI11FERA. 
441 
Tridacna yiyas. —The Giant Tridacna. 
Plate LXXXVI1I. tig. 21. 
Large, white, transversely ovate; with 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 II.—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 impressions. 
Genus 1.—ETHERIA .—Lamarck 
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. 
Etheria elliptica — The Oval Etheria. 
Plate LXXXVIII. fig. 19. 
Oval, flattened, dilated towards the umbones; the apices re¬ 
mote. Inhabits the Indian ocean. 
Genus 2. — CHAM A.— 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. 
Section * Beaks twisted from left to right. ** Beaks turn¬ 
ing from right to left. 
