TRIGONOTRETA. 277 



TRIDENTATE. (Tridentatus.) Having three teeth, or salient 

 points. Ex. Hyalaea tridentata, fig. 226. 



TRIGONA. Schum. ? Triangular species of Cytherea, such as C. 

 laevigata, Triplas corbicula, ventricosa, bicolor, &c Fig. 117 b. 



TRIGONACEA. 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. Fig. 

 139, 140. 



TRIGONAL. Triangular, having three sides. 



TRIGONELLA. Humph. Mactra, Auct. 



TRIGONIA. Brug. (rpiywrov, trigonon, triangular.) Fam. Tri- 

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

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

 and 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 

 grooved sides, two similar teeth in the other ; ligament external, 

 thick ; muscular impressions two in each valve. — Obs. Only one 

 recent species of this marine genus is known, the T. pectinata, 

 which comes from New Holland ; and was formerly so rare, that a 

 much worn odd valve has been sold for a considerable sum. It is of 

 a brilliant pearly texture" within, tinged with purple or golden 

 brown. Fossil species occur in Lias, upper and lower Oolite, 

 and Green-sand. T. Pectinata, fig. 139. 



TRIGONOSEMUS. Konig. A genus composed of species of 

 Terebratula, Auct. which have one valve produced into a beak, 

 perforated, or as it were truncated at the apex. T. lyra, fig. 208, 

 differing from Terebratula lyra, Lam. 



TRIGONOSTOMA. A sub-genus of Helix, with a trigonal aperture. 

 Gray's Turton, p. 139. 



TRIGONOTRETA. Kbnig. A genus composed of species of 

 Terebratula, Auct. which have the hinge of the larger valve pro- 

 duced into a triangular disc, divided by a triangular foramen in 

 the centre. Spirifer, Sowerby, belongs to this genus. Fig. 

 214, 215. 



