280 INVERTEBRATA OF MASSACHUSETTS. 



lips ; outer lip sharp, notched, and a little everted ; pillar twisted, 

 regularly arched above. Length i inch, breadth ^ inch, di- 

 vergence 35. 



Found by Professor Adams in Dartmouth harbour, clinging to 

 marine plants, a few feet below low water, with other species. 



This little shell would hardly be distinguished when mixed with the 

 young of C. Say I. Its color is the same, and it is not unlike it in 

 marking. From the full-grown shell it is readily distinguished by its 

 bulging shape, the apparent jutting of one whorl over another, its 

 smaller size, and by its deeply notched canal. 



GENUS PLEUROTOMA, LAM. 



Shell turreted, generally ribbed; aperture terminating in a 

 straight, more or less elongated canal ; outer lip. at its posterior 

 junction, having a fissure or notch. 



PLEUROTOMA DECUSSA'TA. 



Shell oval, ashy or flesh-colored, with twenty-Jive minute folds, 

 and close revolving lines ; notch of the outer lip shallow. 



FIGURE 185. 



State Coll., No. 272. Soc. Cab., No. 2364. 



Pleurotoma decussata, COUTHOUY ; Bost. Journ. Nat. Hist., ii. 183, pi. 4, f. 8. 



Shell small, ovate, of an ashy-white, or flesh-color, covered 

 with remnants of an olive-colored epidermis ; whorls five or six, 

 convex, the lowest being two thirds the length of the shell, cover- 

 ed with twenty -five to thirty inconspicuous folds or ribs, undulated 

 and oblique in conformity to the outer lip, and vanishing on the 

 convexity of the whorl ; lines of growth regular and distinct, and 

 these, with numerous, elevated revolving threads, make a fine 

 net- work over the whole shell ; spire regularly sloping to an 

 acute point ; suture well marked, with a slight shoulder near it on 

 the whorls ; aperture half as long as the shell, narrow-oval, 

 terminating in a broad and very brief channel ; outer lip sharp, 

 with a shallow recess or notch, as it joins the whorl ; pillar arched, 

 flattened, and smooth ; operculum pear-shaped, with the apex 



