314 HELICHXS:. 



and nothing can be more decisive that it is not founded 

 in natural characters than the fact, that although it is 

 adopted by all authors, yet it is so modified and dismem- 

 bered by each one of them, that no two of them include 

 the same species under it. Many groups have been 

 detached by different authors, some of which will doubt- 

 less prove true genera when the difficult task of examin- 

 ing animals so small shall have been accomplished. 

 Among our own shells which have been arranged under 

 this genus, we have already stated that P. exigua is a 

 Carychium, and belongs to a different family, and pro- 

 bably P. corticaria also. All the other small species 

 may be divided into two groups ; those which have the 

 oral tentacles distinct, (Pupa,) and those in which they 

 are not decidedly apparent, (Vertigo.) This distinction 

 would seem to be only one of degree, were it not that 

 there are peculiarities in the shells of the two groups 

 also ; those of the former group being paler colored, 

 ovate, and more solid, while those of the latter are 

 darker, thin, and more cylindrical ; the aperture of the 

 latter is armed with denticles, which are peculiarly long 

 and slender. The animals are also more aquatic in their 

 habits ; and the want of inferior tentacles would seem to 

 bring them somewhat in alliance with the fresh-water 

 Pulmonata. We are not disposed to insist on the sub- 

 division, however, in the present limited knowledge we 

 have of the structure of the animals belonging to the 

 shells. The large West India species, with which our 

 Florida species, P, incana, is to be associated, consti- 



