274 INVERTEBRATA OF MASSACHUSETTS. 



Odostomia tri'fida. 



Shell small, acute-conic, glossy white, with numerous impressed 

 revolving lines, of which the two uppermost, and those about the 

 base of the last lohorl are most distinct ; aperture narrow. 



Figure 179. 



State Coll., No. 26. Soc. Cab., No. 2370. 



Actos'on trifidus, Totten ; Sillimans Journ., xxvi. 368, pi. 1, f. 4, a, b. 



Shell elevated, pointed, smooth and glossy, of an ivory-white 

 color ; whorls about eight, flat, separated by a sharp, slightly de- 

 pressed suture, on which are from three to five revolving lines, of 

 which the two next below, and the one immediately above, the su- 

 ture are most deeply sculptured ; about the front of the lower 

 whorl are usually ten or twelve very fine lines also ; aperture nar- 

 row, about one third the length of the shell, acutely angular 

 above ; outer lip sharp and thin, sometimes showing within, and 

 on its sharp edge, the impressed lines ; the inner rhargin regularly 

 curved ; the pillar, widening and expanding a hltle, is produced so 

 that an acute angle is formed by the junction of the two lips in 

 front ; about the middle of the inner margin is a single, sharp, ob- 

 lique fold, formed by the revolution of the outer lip within the 

 shell ; operculum horny ; apex nearly terminal, sub-spiral. Length 

 1 inch, breadth yV inch, divergence 23°. 



First found by Colonel Totten on the shores of Rhode Island, 

 adhering to Pecten concentricus. Since then it has been found by 

 Professor Adams among sand from New Bedford harbour. 



Family CJINJILIFERA, Lam. 



Shell with a canal,more or less extended, in front of the aperture; the outer 

 lip of which does not alter its form by age. 



Genus CErItHIUM, Lin. 



Shell elongated, turreted ; aperture short, oblong, oblique, termi- 

 nating in front by a short, recurved canal. 



