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. HIPPOPUS. Valves closed at or near the hinge. Fig. 156. 2. TRIDACNA. An hiatus near the hinge. Fig. 157. TRIDENTATE. (_Tridentatus_.) Having three teeth, or salient points. _Ex._ Hyalæa tridentata, fig. 226. TRIGONA. Schum.? Triangular species of CYTHEREA, such as C. lævigata, Triplas corbicula, ventricosa, bicolor, &c. Fig. 117 _b._ TRIGONACEA. Lam. A family belonging to the order Conchifera 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. ([Greek: trigônon], _trigonon_, triangular.) _Fam._ Trigonata, 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, compressed, 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. König. 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. König. A genus composed of species of Terebratula, Auct. which have the hinge of the larger valve produced into a triangular disc, divided by a triangular foramen in the centre. Spirifer, Sowerby, belongs to this genus. Fig. 214, 215. TRILOBATE. ([Greek: Treis] three; [Greek: lobos], division, lobe.) Divided into three lobes or principal parts. Ex. Malleus, Fig. 165. TRILOCULINA. D'Orbigny. A genus of microscopic Foraminifera. TRIPARTITE. (_Tripartitus_) composed of or divided into three separate parts.