XX INTRODUCTION. [CH. 



species from which it was proposed to separate them. 

 They apparently forgot that the very difference of locality 

 or habitat, with its accompanying conditions, caused the 

 variation in question. 



Monstrosities. These abnormal forms of shells are 

 frequently repeated, and even appear to be hereditary ; 

 and it is not easy at first to distinguish them from varie- 

 ties. Both are probably owing to an irregularity, or an 

 excessive or defective power, of secretion in the mantle 

 of the animal. The monstrosity seldom, if ever, occurs 

 in the first stage of growth ; and the examination of the 

 top whorls or apex of a univalve, or of one of the beaks 

 of a bivalve (being in each case the nucleus of the shell), 

 offers a good criterion to distinguish monstrosities from 

 species. The normal or regular form becomes distorted 

 at a subsequent stage of growth, instead of pursuing the 

 usual course of formation. 



Reversed shells. Among the numerous cases of mon- 

 strosity to which the Mollusca are liable, by reason of 

 their comparatively simple organization, none is more 

 remarkable than the reversed turn and position which the 

 spire of univalve shells and the valves of some Acephala 

 not unfrequently assume. The twist of the spire in 

 univalves is, with a few exceptions, dextral or from left 

 to right, the shell being placed with its spire towards 

 the observer and its mouth downwards. If the shell is 

 held in this position before a mirror, the spire will of 

 course appear to be sinistral or turning from right to 

 left, which is termed "reversed." This phenomenon 

 occurs in most species of Mollusca which have spiral 

 shells ; but it is more rare in some than in others. In 

 certain genera the shell is naturally reversed; and a 

 dextral spire becomes the exception or monstrosity. In 

 a few species the spire is as frequently sinistral as dex- 



