TRIBE TRIDACNACEA. 231 
arranged ina few distant radiating rows; epidermis thicker and 
blackish; nacre of a leadlike irridescence, with green splotches 
and the scars green; margins less fragile and better finished, beak 
Jess developed, callosities fewer. Senegal. } 

ORDER MONOMYARIA. 
A single subcentral muscular impression. 
Section [. 
Tigament marginal, elongated, sublinear. 
Trise TRIDACNACEAs 
Transverse, equivalve, solid ; the muscular umpression situated above 
the centre of the ventral margin, and extended on each side pa- 
rallel to it. 
TRIDACNA. 
Shell regular, equivalve, inequilateral and transverse ; a gaping pos- 
terior space near the beaks ; hinge with two compressed anterior 
inserted unequal teeth ; ligament marginal and external.\ 
T. Gieas. Lam. 1.—Cuama G. Gmel. 3299.—D. p. 213.—Ch. 
J. 495.—Iast. t. 354. f191.— FE. t. 235. f. 1. Very large, pon- 
derous transversely ovate, with great imbricated and scaly ribs; the 
scales short, thick, arcuated and crowded ; interstices of the ribs not 
striated, except in young specimens ; inner margin of the Junule 
crenulated. Indian Ocean. 
T. Etoneata Lam. 2. Ovate oblong, the posterior side the 
mote produced, and its aperture large, dorsal margin crenated ; ribs 
with numerous imbricated half raised scales—Var. A. Whitish, the 
interstices of the ribs obsoletely striated—Var. B. (Gual. t. 92. E.) 
Yellowish white, the interstices longitudinally striated—Var. C. 
(E. t, 235. f. 4.) Whitish, both interstices and ribs, longitudinally 
striated near the beaks.—63. Indian Ocean. 
T. Seuamosa Lam. 3.—Cuama Giecas. var. a. D p. 213.-——Ch. 
f. 1997, 8.—Kn. 1. t. 19. f. 83.—C. Gieas. W. t. 9. f. 2. Ovate, 
white, reddish when young, ribs with large erect and distant scales ; 
interstices thickly striated; posterior depression small, with the in- 
ner margin crenated. Indian Ocean. 

1 See too the E. DenuAmrin the appendix to Denham and Clap- 
perton’s Travels. 
