274 TOMELLA. 



THELIDOMUS. Sw. A generic name under which Swainson has 

 described a division of the, genus Helix, and which he has also used 

 to designate a genus in the family of " Rotellinse," founded upon 

 an aggregate of loose particles collected and agglutinated in a 

 spiral form by the larva of an insect. Sw. Malac. p. 330 and 353. 



THEMEON. Montf. A genus of microscopic Foraminifera. 



THEODOXUS. Montf. A division of the genus Nerita. Fig. 324, 

 N. virginea. 



THETIS. Sow. (A sea nymph) A genus of fossil shells, de- 

 scribed 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. Lithophagidae, Lam. Pyloridea, Bl. 

 A genus described as intermediate between Anatina, and Mya, 

 and in some degree resembling Corbnla. 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 Amphidesm a flexuosa, 

 Lam. and similar species, belonging more properly to the genus 

 Lucina. 



TIARA. Sw. A genus of "Mitranse," Sw. thus described: "Aper- 

 ture narrow, linear, or of equal breadth throughout ; outer lip 

 andbaseof 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 Muricidce and 

 Cymbiola." Sw. Malac. p. 319. The principal differencebetween 

 Tiara and Mitra appears to be that in the latter, the aperture is 

 more linear and contracted in the centre. Mitra Episcopalis 

 s an example. 



TINOPORUS. Montf. A genus of microscopic Foraminifera. 



TIRANITES. Montf. A division of the genus Baculites. 



TOMELLA. Sw. A genus of " Pleurotominre," Sw. thus de- 



