GASTEROPODA. 271 



rocks ; they have been compared to elongated pleurotomarice, 

 or to cerithia with notched a]3ertures ; the first suggestion is 

 most probably correct. 



Fossil, 50 species. L. Silurian — Permian. North America, 

 Europe. 



Trochotoma, Lycett. 



Etymology, Trochus, and tome, a notch. 



Synonym, Ditremaria, D'Orbigny. 



Type, T. conuloides, PI. X., Pig. 26. 



Shell trochiform, slightly concave beneath; whorls flat, 

 spirally striated, rounded at the outer angles ; lip with a single 

 perforation near the margin. 



Fossil, 10 species. Lias — Coral Eag. Britain, Prance, &c. 



? Cirrus, Sowerby. 



Etymology, cirrus, a curl. 



Type, C. nodosus, Sby. Min. Con. t. 141 and 219. 



Shell sinistral, trochiform, base level ; last whorl enlarging 

 rather more rapidly, somewhat irregular. 



Fossil, 2 species. Inf. oolite, Bath oolite. Britain, Prance. 



This genus was founded on a jpleurotomaria, a euomphalus, 

 and C. nodosus. (v. Min. Con.) It is still doubtful what 

 species may be referred to it. 



Fig. 117.* 



Iaj«"THINA, Lam. Yiolet-snail. 



Etymology, ianthina, violet-coloured. 



Type, helix ianthina, L. (I. fragilis, Lam.) PL X., Pig. 27. 



Shell thm, translucent, trochiform; nucleus minute, styhform; 

 sinistral ; whorls few, rather ventricose ; aperture four-sided ; 

 columella tortuous ; lip thin, notched at the outer angle. Base 

 of the shell deep violet, spire nearly white. 



Animal head large, muzzle-shaped, with a tentacle and eye- 



* Pig. 117. Ianthina fragilis, Lam. (from Quoy and Gaimard). Atlantic, a, raft 

 6, egg capsules ; c, gills ; d, tentacles and eye-stalks. 



