HELIX. 113 



terms of the definition of the next division, it is 

 perhaps scarcely necessary. The aperture of many 

 of the species in the first family is so very simi- 

 lar to that of Trochi, that in arranging them we 

 must be careful to observe the convexity of the co- 

 lumella : this sufficiently distinguishes the Helices. 

 The general character, — which is obvious in al- 

 most 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 j 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, eXt^, implies merely a shell constructed 

 with a spire or with circumvolutions of the whorls. 

 It might therefore include several other genera. 



