TRACHELIPODA. Lam. ([Greek: trachêlos], _trachelos_, a neck; [Greek: poda], _poda_, foot.) The third order of the class Mollusca, in the system of Lamarck. The trachelipodous mollusca are described as having the posterior part of the body spirally twisted and separated from the foot; always enveloped in a shell. The foot is free, flat, attached to the base of the neck. Shell spiral, and enclosing the animal when at rest. This order contains the families, Colimacea, Lymnacea, Melaniana, Peristomiana, Neritacea, Janthinea, Macrostomata, Scalariana, Plicacea, Canalifera, Alata, Purpurifera, Columellaria, Convolutæ. The genera belonging to these families, are represented in the plates, fig. 264, to 462. TRANSVERSE. (Crosswise.) A shell is said to be transverse, when its width is greater than its length, that is, when it is longer from one side to the other than from the umbones to the ventral margins. The term is applied by some authors to express the direction of the lines of growth in bivalve shells, and the spiral lines in spiral shells. See CONCENTRIC. TRAPEZIUM. Meg. CYPRICARDIA, Lam. TRAPEZIFORM, or TRAPEZOID. ([Greek: trapezion], _trapezion_, _trapezium_; [Greek: eidos], _eidos_, form.) Having four unequal and unparallel sides. _Ex._ Cucullæa, fig. 133. TRIBULUS. Klein. RICINULA, Lam. TRICHOTROPIS. Brod. and Sow. ([Greek: Trichos], _trichos_, hair; [Greek: tropis] _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; operculum 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, 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. 157.