296 TRICJONIA. 



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

 large byssiis, by which the animal fixes itself to marine sub- 

 stances ; 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. PI. ix. fig. 157. 

 TEIDACNACEA. 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 undiilated ventral margin. This family contains the 

 genera : 



1. Hippopus. Valves closed at or near the hinge. Fig. 156. 



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

 TEIDENTATE. {Tridenfatus.) Having three teeth, or salient 



points. Ex. Hyalsea tridentata. Eig. 226. 



TEIGONA. Schum. ? Triangular species of Ctthebea, such as 

 C. laevigata, Triplas corbicula, ventricosa, bicolor, &c. The 

 author found, however, in preparing the monograph of Cythersea, 

 No. 12, Thesaurus Conchyliorum, that the triangular passed by 

 so many gradations into the rounded or oval forms, that it was 

 impossible to find a resting place. PI. vi. fig. 1176. 



TEIGONOC^LITJS. D'Nyst, 1835. A genus of bivalve shells, 

 resembling Pectunculus, but distinguished by a triangular pit in 

 the area of the hinge. Ex. T. aurita. PL vii. fig. 136. 



TEIGONACEA. Lam. A family belonging to the order Conchi- 

 fera Dimyaria, containing the genera Trigonia and Castalia, the 

 latter of which ought to be removed to the Nayades. Eig. 139, 

 140. 



TEIGONAL. Triangular, having three sides. 



TEIGONELLA. Humph. Mactea, Auct. 



TEIGONIA. Brug. (rpiywvov, trigonon, triangular.) Fam. Tri- 

 gonata. Lam. Camacea, Bl. — Bescr. Equivalve, inequilateral, 

 transverse, sub- trigonal, costated and granulated without, pearly 



