282 INVERTEBRATA OF MASSACHUSETTS. 



This is not likely to be confounded with any other of our shells. 

 Its two revolving ridges mark it well. It is interesting as being the 

 first species of the genus found in our northern Atlantic waters. 

 Neither of our three species belongs, unequivocally, to this genus ; 

 they approach very near to Fusus. Still, the direction of the lines of 

 growth indicates the sinus in the lip to be constant ; and on this rests 

 their claim to a place in the genus Pleurotoma. 



Pleurotoma plicata. 



Shell small, cinereous, ovate ; whorls six, reticulated with prom- 

 inent, longitudinal ribs, and elevated, revolving lines ; sinus of the 

 lip distinct. 



Figure 187. 

 State Coll., No. 274. Soc. Cab., No. 2366. 



Pleurotoma plicata, Adams ; Bost. Journ. Nat. Hist., iii. pi. 3, f. 6. 



Shell small, elongated-ovate, somewhat turreted, of an ashy- 

 white color ; whorls six, the lowest one about two thirds the 

 length of the whole shell, and bearing about twelve prominent, 

 somewhat oblique, rib-like folds, which are crossed by ten or 

 more elevated, revolving threads, rendering the ribs a Httle nodu- 

 lous ; the other whorls form a very pointed, somewhat turreted 

 spire, on which the ribs and revolving lines are continued. Aper- 

 ture narrow, less than half the length of the shell ; outer lip 

 greatly thickened by one of the ribs, the notch at its posterior 

 part being deep, distinct, and smooth. Length \ inch, nearly, 

 breadth j%- inch, divergence 45'^. 



Found in mud from New Bedford harbour, by Professor C. B. 

 Adams. 



This species is of about the same size and shape as P. decusscita, but 

 is distinguished by the much more conspicuous folds, which run the 

 entire length of the whorl ; and the revolving lines also are much 

 more distinct, and fewer in number. The canal is very short. 



Gencs CANCELLARIA, Lam. 



Shell ovate, turreted, cancellated ; canal partial, very short or 

 wanting ; pillar plaited, the folds nearly transverse. 



