302 INVERTEBRATA OF MASSACHUSETTS. 



olate, and often with bands of white or yellow, of different 

 widths ; surface more or less wrinkled and encircled with numer- 

 ous, coarse, unequal ridges. Some are nearly smooth, and others, 

 at the lines of growth, have series of raised, concave scales, which 

 render the whole shell rough and prickly to the touch ; whorls 

 five or six, in some convex, so that the spire appears turreted, in 

 others flattened below the sutures, so as to be pyramidal. Aper- 

 ture oval, the outer lip regularly curved, sharp, but thickened, and 

 armed with blunt teeth at a little distance within, so that the aper- 

 ture appears spreading ; the pillar lip moderately flattened, 

 smooth, its lower portion a little twisted, so as to form a moder- 

 ate 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. Common length 1J inch, 

 breadth y^ inch. 



Inhabits the ocean rocks everywhere. 



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

 recognised, yet it is infinitely varied in its details. All specimens have 

 the coarse, revolving ridges, and the peculiar twist of the flattened pil- 

 lar, characteristic of the genus. They may be divided 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 rhomboidal, rather than an ovate outline. This 

 smoothness is not the effect of age, as has been generally stated, for 

 the young shells, in both groups, are like the old. In these there is 

 nothing like an umbilicus. 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, scolloped edges of 

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

 youngest specimens. In these the callus is abundant upon the pillar, 

 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 aper- 

 ture, that the two lips are continuous, and their junction rounded. The 

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

 Lamarck's species P. imbricata. 



As to coloration, both varieties pass from white through yellow 

 to a dark chocolate or slate-color ; but specimens of the first group, 



