276 TRIDACNACEA. 



TRAPEZIUM. Meg. Cypricardia, Lam. 



TRAPEZIFORM, or 



TRAPEZOID. {rpaive'CLov, trapesion, trapezium', sicog, eidos, form.') 

 Having four unequal and unparallel sides. Ex. Cucullsea, fig. 

 133. 



TRIBULUS. Klein. Ricinula, Lam. 



TRICHOTROPIS. Brod, and Sow. {T P i X oq, trichos, hair ; rpoTrtc, 

 tropis, keel.) Fam. Purpurifera, Lam. — Bescr. Turbinated, 

 keeled, thin, umbilicated ; aperture longer than the spire, entire j 

 columella obliquely truncated ; outer lip thin, sharp ; epidermis 

 horny, produced into long hairs at the angles of the shell ; oper- 

 culum horny, with the nucleus lateral.— Obs, Although the shells 

 of this genus have something of the shape of Turbo, they are 

 distinguished from that genus at once by the thinness of the shell. 

 They are also known from Buccinum, by the absence of a canal. 

 Only two or three species are known, which belong to the Northern 

 and Arctic Oceans. T. bicarinata, fig. 429. 



TRIDACNA. Auct. Fam. Tridacnacea, Lam. Chamacea, Bl. — ■ 

 Descr. Equivalve, regular, inequilateral, radiately ribbed, adorned 

 on the ribs with vaulted foliations, waved at the margins, with a 

 large, anterior hiatus close to the umbones, for the passage of a 

 large byssus, by which the animal fixes itself to marine substances; 

 hinge with a partly external ligament ; two laminar teeth in one 

 valve, one in the other. — Obs. The beautiful shells composing 

 this genus are of a delicate white colour, tinged with buff. One 

 species, theT. gigas, attains a remarkable size, measuring from two 

 to three feet across, and weighing five hundred pounds. Tridacna 

 is distinguished from Hippopusby the large opening in the hinge. 

 T. elongata, fig. 157. 



TRIDACNACEA. Lam. A family belonging to the first section 

 of the order Conchifera Dimyaria, Lam. described as regular, 

 equivalve, solid, and which are remarkable for the deeply sinuated 

 or undulated ventral margin. This family contains the genera : 



1. Hippo pus. Valves closed at or near the hinge. Fig. 156. 



2. Tridacna. An hiatus near the hinge. Fig. 157. 



