THETIS. Sow. (_A sea nymph._) A genus of fossil shells, described as resembling Mactra, but not having the internal ligament, and having several small, acuminated, cardinal teeth, but no lateral teeth. It resembles Tellina in some degree, but has not the posterior fold. THIARELLA. Sw. A sub-genus of Mitra, Lardn. Cyclop. Malac. p. 319. THRACIA. Leach. _Fam._ Lithophagidæ, Lam. Pyloridea, Bl. A genus described as intermediate between Anatina, and Mya, and in some degree resembling Corbula. T. corbuloides, fig. 93. THUNDER-STONES. One of the vulgar appellations which have been applied to shells of the genus Belemnites. THIATYRA. Leach. A genus composed of AMPHIDESMA _flexuosa_, Lam. and similar species, belonging more properly to the genus LUCINA. TIARA. Sw. A genus of "Mitranæ," Sw. thus described: "Aperture narrow, linear, or of equal breadth throughout; outer lip and base of the body whorl contracted, the former generally striated; an internal canal at the upper part of the aperture; shell (typically) turrited, and equally fusiform; representing the _Muricidæ_ and Cymbiola." Sw. Malac. p. 319. The principal difference between Tiara and Mitra appears to be that in the latter, the aperture is more linear and contracted in the centre. Mitra Episcopalis is an example. TINOPORUS. Montf. A genus of microscopic Foraminifera. TIRANITES. Montf. A division of the genus Baculites. TOMELLA. Sw. A genus of "Pleurotominæ," Sw. thus described: "Fusiform, smooth; the spire of very few whorls, and not longer than the channel; inner lip with a thick callosity at the top; the slit short and wide; lineata, En. Méth. 440, f. 2, clavicularis, Ib. f. 4. filosa. En. Méth. 440, f. 6. lineolata. Ib. f. 11." Sw. p. 314. TOMOGERUS. Montf. ANASTOMA, Auct. Fig. 471. TONICHIA. Gray. Syn. B. M. p. 126. A genus composed of those species of Chiton which have the margin smooth. TORNATELLA. Auct. _Fam._ Plicacea, Lam.--_Descr._ Oval, spirally grooved; spire short, rather obtuse, consisting of few whorls; aperture long, narrow, rounded anteriorly; outer lip simple; inner lip thin, slightly spread, columella spiral, incrassated, confluent with the outer lip. The recent species are few. Several fossil species occur in London Clay, Inferior Oolite and Calcaire-grossièr. Monoptygma, Lea, resembles this genus, but has a fold on the inner lip. Fig. 343, T. solidula. TORTUOUS. (_Tortuosus_) Twisted. This adjective is sometimes applied as a specific name; as Arca tortuosa.