328 PYEAMIDELLID^. 



lines of growth scarcely perceptible ; whorls five or six, separated 

 by a well-defined suture ; and, in most specimens, a distinct line 

 revolves just before the suture, giving the appearance of a 

 doul )le suture ; the lowest whorl is proportionally larger than 

 the others, and constitutes about half the length of the shell ; 

 aperture oval, outer lip sharp and simple ; pillar lip bluish- 

 white, smooth and rounded ; a transverse, white fold is formed 

 by the turning of the lip within the shell, before which it is 

 a little raised and turned outwards, producing an umbilical 

 chink, and is extended so as to form a considerable project- 

 ing angle at the lower extremity ; operculum horny. Length, one 

 fifth of an inch ; breadth, one tenth of an inch ; divergence, twenty- 

 eight degrees. 



First found by Mr. Couthouy, at Chelsea, near the ferry landing, 

 adhering to decaying wood. It has since been found in various sim- 

 ilar situations, and under the damp portions of loose stones left on 

 the shore at low tide. 



It is distinguished from O. frijida, with Avhich it would be con- 

 founded without intimate examination, by its being a less slender 

 and pointed shell, the disproportion of the last whorl, and the 

 greater convexity of all the whorls, and the want of two or three 

 revolving lines so characteristic of O. trifida. In the latter shell the 

 fold of the columella is oblique, and in O. bisiiturcdis it is nearly 

 transverse. It will also be necessary to compare it with the next 

 species. I should be disposed to regard it as the same with Turbo 

 nnidentatus, Montagu (Test. Brit. 324) ; but as it is impossible to 

 decide on such minute species with no obvious characteristics, from 

 description only, I must leave it unsettled. 



It is almost certainly the Turritella bisuluralis of Say ; at least, 

 I know of no other shell which will at all answer to his description. 

 The figure and description of Rissoa rvpestris, Forbes, also corre- 

 spond with it. 



Odostomia trifida. 



Fig. 179. 



Shell small, acnte-conic, glossy white, with numerous impressed revolving 

 lines, of wliich the two uppermost, and those about the base of the last whorl, 

 are most distinct ; aperture narrow. 



Artn'on trifidiis, Totten, Sillim. Jonrn. xxvi. 3G8, pi. 1, figs. 4, a, b. 

 Odostomia tri/iild, Gould, Iiiv. 1st cd. 274, fi<,^ 179. — De Kay, N. Y. Moll. 114, pi. 8, 

 fig. 170. — Stimpson, Check Lists, 5. 



