HELIX. 93 



others prevail over a broad extent in every direction. 

 These hints are sufficient to show the direction which 

 should be given to our investigations, before we can come 

 at the true history and natural classification of the snails. 

 For reasons already given, we would not attempt a 

 definitive arrangement of our whole list on the ground of 

 natural alliance. But as the number of species is such 

 as to require some kind of systematic arrangement for 

 convenience' sake, the divisions and subdivisions in the 

 following table are adopted ; although unfortunately, in 

 several instances, it separates species from each other 

 which are naturally allied. This arrangement supposes 

 the shells to have arrived at maturity ; and some little 

 difficulty might arise, practically, to those not familiar 

 with the differences which often exist between the young 

 and adult shell, were these differences not pointed out. 

 Immature shells always have a simple, sharp lip, Avith 

 the epidermis a little in advance of the calcareous por- 

 tion ; they are always destitute of teeth or laminte in 

 the aperture ; many of them have an angular periphery, 

 and an open umbilicus, in species where such an opening 

 does not appear in the adult shell. As maturity comes 

 on, the lip becomes thickened, in many species widely 

 reflected, and then armed with teeth. An abundant 

 callus is deposited, which in many instances covers over 

 and closes the umbilical opening ; and as the last whorl ap- 

 proaches the aperture it loses the angular character which 

 distinctly marked the shell in its earlier stages. [G.] 



