80 



DISTINCTION OF FEESHWATER 



[Ch. Ill 



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

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



Fig. 42. 



Fig. 43. 



Fig. 44 



Neritina globulus. Paris basin. 



Nerita granulosa. Paris basin. 



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 Neritce^ the 

 inner margin of the outer lip toothed or crenulated. (See 

 fig. 43.) 



A few genera, among which Cerithiuin (fig. 44) is the most 

 abundant, are common both to rivers and the sea, having spe- 

 cies peculiar to each. Other genera, like Auricula (fig. 38), are pa^^g basin 

 amphibious, frequenting marshes, especially near the sea. 



The terrestrial shells are all univalves. The most abundant genera 

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

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



Fig. 45, 



Helix Turonenais. 

 Faluns, Touraine. 



Fig. 46. 



Fig. 47. 



48. 



Pupa Clausilia 

 tridens. Mdens. 

 Loess, Loess. 



Fig. 49. 



Bulimus luhricus. 

 Loess, Eliine. 



Fig. 50. 



and Achatlna ; 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 oonchologists 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- 



^.. .......... ... >,.^. , ^^"^<^5 which has a slight indentation, have entire 



from the Jumna. moutlis ; 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, 



