PURPURA. 



361 



below the sutures, so as to be pyramidal ; aperture oval, the outer 

 lip regularly curved, sharp, but thickeued, aud armed with blunt 

 teeth at a little distance within, so that tlie aperture appears spread- 



Fig. 630. 



P. lapillus. 



ing ; the pillar lip moderately flattened, smooth, its lower portion a 

 little twisted, so as to form a moderate projection within the shell, 

 and a crescent-shaped umbilical depression outside ; canal short, 

 turning a little to the right ; throat generally light reddish-brown, 

 with a lighter border to the lip. Operculum horny, elliptical. Com- 

 mon length, one and one fourth inches; breadth, seven tenths of an 

 inch. 



Inhal)its the ocean rocks everywhere, from Greenland through 

 all New England. 



While there is an individuality about this shell, by which it is 

 easily recognized, yet it is infinitely varied in its details. All speci- 

 mens have the coarse, revolving ridges, and the peculiar twist of 

 the flattened pillar, characteristic of tlie genus. They may be di- 

 vided into two groups, those with a smooth, and those with a rasp- 

 like surface. The smooth shells are the most solid, and are usually 

 flattened near the suture, so as to give the shell a rhomlioidal, 

 rather than an ovate outline. This smoothness is not the effect of 

 age, as has lieen generally stated, for the young shells, in both 

 groups, are like the old. In these there is nothing like an umbili- 

 cus. These are the true P. lapillus. 



In the other group, the whorls are more convex, the suture deep, 

 and the surface is rendered rasp-like by the sharp, scalloped edges 

 of the successive lines of growth, which are most conspicuous in the 

 youngest specimens. In these the callus is abundant upon the pil- 

 lar, and rises in such a manner as to seem to cover an umbilicus. 

 In some specimens it is so abundant at the posterior angle of the 

 aperture, that the two lips are continuous, and their junction round- 

 ed. The canal in this variety is more decided and longer. This 



