HELIX. 
101 
perhaps scarcely necessary. The aperture of many 
of the species in the first family is so very similar 
to that of Trochi, that in arranging them we must 
be careful to observe the convexity of the columella : 
this sufficiently distinguishes the Helices. 
The general character, — which is obvious in 
almost all the species, whether terrestrial, fluviatile, 
or marine, — of thinness and transparency, is of 
great assistance in acquiring a thorough knowledge 
of the genus. In this property many of the Bullss 
partake ; but they are not likely to mislead, on ac- 
count of other generic distinctions. It may be 
remarked, that the greater number of shells which 
are not oceanic, are far more fragile and diapha- 
nous, than those which have to endure the rough 
beating of a boisterous sea. Those which are found 
in still ponds and muddy ditches are, many of 
them, scarcely able to resist the slightest pressure. 
The name, eX/g, implies merely a shell con- 
structed with a spire, or with circumvolutions of 
the whorls. It might therefore include several 
other genera. 
