EXTERNAL FORM OF SHELLS. 415 



fectly regular so long as they continue free. This explains 

 why irregular shells are more rare among the univalves than 

 among the bivalves, as not more than three or four genera 

 of the former ever became attached. Good examples of 

 shells which are irregular when full grown, although regular 

 in their very young state, may be seen in the genera Ostrea, 

 Chama, Hinnites, Magilus, and Vermetus. The very young 

 shell of Chama arcinella, which closely resembles in form a 

 minute Petricola, is frequently found persistent and consti- 

 tuting the apex of the umbones of adult specimens ; and so 

 regular is its form, that I have little doubt, were a concholo- 

 gist to meet with a very young free specimen among the 

 sand of the West Indian coasts, that he would refer it either 

 to the latter genus, or to the genus Cardita. In like manner 

 the very young shell of Hinnites pusio, when persistent in 

 the umbones of the adult, cannot be distinguished from a 

 free regular Pecten. 



Many univalves exhibit the same phenomenon : the young 

 Spiroglyphus and Magilus, as will hereafter be described, 

 are quite regular so long as they remain free ; and the apices 

 of all the Vermeti and Siliquarise show that they also are 

 regular in their youth. The apex of the former has indeed 

 been mistaken for a regular spiral shell, and described as a 

 Turritella by Lamarck and by Dr. Turton. 



Some land shells (for it is only in such shells that I have 

 observed it) offer a very curious anomaly in their form ; 

 they are quite regular in their young state, but change the 

 direction of their last whorls as they approach maturity, and 

 in some cases even reverse the position of the mouth. A 

 remarkable instance of this change of direction occurs in a 

 rather common Brazilian snail, which is transversely stri- 

 ated in its young state, and exactly resembles a common 

 umbilicated snail; but acquires when adult a smooth last 

 whorl much larger than the others, and pressed towards the 

 side next the mouth, by which means the axis is bent out 

 of the line, and the umbilicus is compressed and closed. 

 This obliquity in the form of its whorls gives the shell the 

 appearance of having been slightly crushed, from which cir- 

 cumstance it has been called by Baron Ferussac Helix con- 

 tusa. A similar departure from the regular form is found, 

 in even a greater degree of development, in a minute species 

 from the same country, named Helix deformis (Wood's Sup- 

 plement, tab. 7. fig. 40) ; and may also be seen in a slight 

 degree in the Helix concamerata of Wood's Supplement, tab. 

 7, fig. 21. The last whorl of Bulimus lyonettianus is com- 

 pressed on the side opposite to the mouth, and produced into 



