416 EXTERNAL FORM OF SHELLS. 



an acute angle, which gives the shell a very anomalous 

 appearance. A somewhat analogous deformity, but not so 

 much developed, may also be observed in Helix auris- 

 leporis. This kind of variation is, however, not confined 

 to the inoperculated land shells, for the Cyclostoma com- 

 pressum, figured by Mr. Wood in his Supplement, tab. 6, 

 fig. 42, differs from all others of the genus in its last whorl 

 being compressed, although not in so great a degree as in 

 the Bulimus lyonettianus ; and in the Cyclostoma tortum 

 (Turbo tortus of Wood's Supplement, tab. 6, fig. 32), the 

 last whorl is slightly pressed on one side like that of Helix 

 contusa, the mouth being produced in front, and nearly in 

 a line with the axis. 



But the most remarkable of these anomalies occurs in the 

 genus Anastomus, where the mouth of the young shell occu- 

 pies its ordinary situation in front of the whorls, the animal 

 in this early stage doubtless crawling in the usual manner, 

 with the spire of the shell uppermost ; but as it approaches 

 maturity, the end of the last whorl is curved upwards, and 

 the mouth of the perfect shell is applied to the upper edge 

 of the last whorl but one, with the opening directed towards 

 the tip of the spire in such a manner that, in walking, the 

 animal must crawl with the tip of the spire downwards, and 

 thus completely reverse the position of its shell. A simi- 

 larly formed shell has been discovered in the fossil state, and 

 named Strophostoma ; and this from the roundness of its 

 mouth, has been supposed to belong to the family of Cyclo- 

 stomidae. 



The axis of most spiral shells is perfectly straight, but the 

 species of some few genera, such as Eulima and Sty lifer, are 

 very liable to have it more or less curved or twisted. And 

 this is not the only point of resemblance between these two 

 genera, both having the same polished surface and similar 

 varices on the spire ; but they differ in the degree of solidity 

 of their shells. The Styliferi live buried in the substance of 

 Star-fish, which may perhaps account for their inflections : 

 the habits of the Eulimse are unknown. 



The growth of other spiral univalve shells appears never to 

 be thrown out of its proper course, except by some accident, 

 such as the interruption caused by the occasional attachment 

 of a foreign body, or by a fracture. I have a Fusus virgo 

 and a F. colosseus, which are thus bent ; and there are two 

 specimens of the common Whelk in the British Museum, the 

 spires of which are very much elongated, the elongation 

 having evidently been caused by a fracture in their very 

 young state. One of the latter has been described as a dis- 



