TRIBE TRIDACNAOEA. 231 



arranged in a 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 

 less developed, callosities fewer. Senegal. ] 



ORDER MONOxMY/VRIA. 



A single subcentral muscular impression. 



Section I. 

 Ligament marginal, elongated, sublinear. 



Tribe Tridacnacea. 



Transverse, equivalve, solid ; the muscular impression 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, j 



T. Gigas. Lam. 1.— ChamaG. Gmel. 3299.— B. p. 213.— Ch. 

 f. 495.— List. t. 354./. 191.— i?. t. 235./. 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 lunule 

 crenulated. Indian Ocean. 



T. Elongata 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./. 4.J Whitish, both interstices and ribs, longitudinally 

 striated near the beaks. — 6|. Indian Ocean. 



T. Squamosa Lam. 3. — Chama Gigas. var. a. Dp. 213. — Ch. 

 f. 1997,8.— Kn. 1. t. 19./ 3.— C. Gigas. TV. t. 9./ 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. Denhamiui the appendix to Denham and Clap- 

 perton's Travels. 



