TROCHUS. 517 



There are two marked varieties of this common shell, 

 which differ from each other in shape, and the consequent 

 dilatation or contraction of the profound umbilicus. The 

 one is rather bluntly conic, with a very small perforation ; 

 the other is much less elevated, expanded at the base, 

 and with the aperture of the umbilicus commensurately 

 enlarged ; the latter form much resembles the young state 

 of the former, but vies with adult specimens in size. 



The whorls are six in number, well defined, but only 

 slightly convex ; in some of the flattened specimens, 

 indeed, a slight concavity is perceptible towards, but not 

 adjacent to, the suture ; each volution is, as it were, dis- 

 tinctly raised out of the preceding one. The apex is 

 extremely fine and small, and is very often tinged with 

 yellow or a warm chestnut colour ; the increase of length 

 in the gy ration is rather quick. The base is more or less 

 flattened, and the basal outline is consequently more or 

 less angulated. 



Its sculpture, and its peculiar style of painting, form its 

 most salient characteristics. The shell is strong, opaque, 

 and rather dull, or but little glossy, with the surface 

 roughened by several spiral belts, that vary greatly in 

 number and thickness, but are always narrower than their 

 interstices. There are generally from six to nine upon the 

 penult whorl. These spiral lines are continued upon the 

 base, where they become more approximate and less 

 prominent. The interstitial spaces are smooth, or almost 

 so. The entire exterior, which is greyish, cinereous, or 

 yellowish ash-colour, is obliquely traversed by flexuous 

 linear markings of iron- or slate-grey, smoke-colour, or 

 more rarely of reddish brown, which are closely disposed, 

 lun in an opposite direction to the lines of growth, and do 

 not become confluent, except occasionally upon the base 



