30 



DISTINCTION OF FRESHWATER 



[Ch. III. 



Some naturalists include Neritina (fig. 42) and the marine Nerita 

 (fig. 43) in the same genus, it being scarcely possible to distinguish 



Fie. 42. 



Fig. 43. 



Fie. 44. 



Neritina globulus. Paris basin. Nerita granulosa. Paris basin. 



the two by good generic characters. But, as a general 

 rule, the fluviatile species are smaller, smoother, and more 

 globular than the marine ; and they have never, like the 

 JVeritce, the inner margin of the outer lip toothed or crenu- 

 lated. (See fig. 43.) 



The Potamides inhabit the mouths of rivers in warm 

 latitudes, and are distinguished from the marine cerithia 

 by their orbicular and multispiral opercula. The genus 

 auricula (fig. 38) is amphibious, frequenting swamps and 

 marshes within the influence of the tide. 



The terrestrial shells are all univalves. The most abundant genera 

 among these, both in a recent and fossil state, are Helix (fig. 45), Cy- 

 r.lostoma (fig. 46), Pupa (fig. 47), Clausilia (fig. 48), Bulimus (fig. 49), 



JJedx Turonensis, 

 Faluns, Touraine. 



r. 47. 



4a 



Pupa Clausilia 

 tridens. bidens. 

 Loess. Loess. 



Fig. 49. 



Bulimus lubricus. 

 Loess, Ehine. 



and Achatina ; which two last are nearly allied and pass into each other. 

 The Ampullaria (fig. 50) is another genus of shells, inhabiting rivers 

 and ponds in hot countries. Many fossil species have 

 been referred to this genus, but they have been found 

 chiefly in marine formations, and are suspected by 

 some eonchologists to belong to JVatica and other ma- 

 rine genera. 



All univalve shells of land and freshwater species, 

 with the exception of Melanopsis (fig. 41), and Acha- 

 tina, which § has a slight indentation, have entire 

 mouths ; and this circumstance may often serve as 

 a convenient rule for distinguishing freshwater from marine strata ; 

 since, if any univalves occur of which the mouths are not entire, we 

 may presume that the formation is marine. The aperture is said to be 

 entire in such shells as the Ampullaria and the land shells (figs. 45 — 

 49), when its outline is not interrupted by an indentation or notch. 



Ampullaria glauca, 

 from the Jumna. 



