TRIGONIA. 107 
fera, Alatze, Purpurifera, Columellaria, Convolute. Fig. 264 
to 462. 
TRANSVERSE, (Crosswise,) A shell is said to be transverse 
when its width is greater than its length. 
TRAPE’ZIUM. Meg. Cypricarpia, Lam. 
TRAPE’ZIFORM or 
TRAPE’ZOID. (rpameéwv, trapexion, trapezium; «dos, form.) 
Having four unequal and unparallel sides. Ex. Cucullea, Fig. 
133. 
TRIBULUS. Klein. Ricinuta, Lam. 
TRICHO’TROPIS. Brod. and Sow. (rpuyos, trichos, hair; rpoms, 
tropis, keel.) Fam. Purpurifera, Lam. Descr. Turbinated, 
keeled, thin, umbilicated, aperture longer than the spire, entire ; 
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 
acanal. Only two or three species are known, these belong to 
the Northern ocean. T. bicarinata, fig. 429. 
TRIDACNA. Lam. Fam. Tridacnacea, Lam. Camacea, BI. 
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 
on 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, the T. gigas, attains a 
remarkable size, measuring from two to three feet across, and 
weighing five hundred pounds. Tridacna is distinguished from 
Hippopus, by the large opening in the hinge. T. elongata, fig. 
15 
TRIDACNA’CEA. Lam. A family belonging to the first section 
of the order Conchifera Dimyaria, containing the genera Tri- 
dacna and Hippopus. Fig. 156, 157. 
TRI'GONA. Schum? Triangular species of Cythera, such as 
C. leevigata, corbicula, ventricosa, bicolor, &c. Fig. 117, b. 
TRIGONAL. Triangular, having three sides. 
TRIGONA’TA. Lam. A family of the order Conchifera Dimya- 
ria, Lam. containing the genera Trigonia and Castalia. 
TRIGO'NIA. Lam. (rpryevor, trigonon, triangular.) Fam. Trigo- 
nata, Lam. Camacea, Bl. Descr. Equivalve, inequilateral, trans- 
verse, sub-trigonal, costated and granulated without, pearly, 
iridescent within, denticulated on the inner margin, rounded 
anteriorly, truncated posteriorly ; hinge with four oblong, com- 
pressed, diverging teeth in one valve, receiving between their 
