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 Bullae 

 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, s'X/g, implies merely a shell con- 

 structed with a spire, or with circumvolutions of 

 the whorls. It might therefore include several 

 other genera. 



