46 



LYELL'S ELEMENTS OF GEOLOGY. 



Distinction of Freshwater from Marine Formations. 



Fi 42 to distinguish tne two y Sd generic characters. 

 But, as a general rule, the fluviatile species are small- 

 er, smoother, and more globular than the marine; 

 and they have never, like the Neritse, the inner mar- 



fin of the outer lip toothed or crenulated. (See 

 ig. 41.) 



A few genera, among which Cerithium (Fig. 42.) 

 is the most abundant, are common both to rivers and 

 the sea, having species peculiar to each. Other 

 genera, like Auricula (Fig. 36.), are amphibious, liv- 

 ing both in freshwater and on land. 



The terrestrial shells are all univalves. The most 

 abundant genera among these, both in a recent and 

 fossil state, are Helix (Fig. 45.), Cyclostoma, Pupa, 

 (Fig. 44.), Clausilia, Bulimias (Fig. 43.), and Acha- 

 ^ na which two last are nearly allied and pass into 

 each other. The same may be said with almost 

 equal truth of Pupa and Clausilia. 

 Fig. 43. Fig. 44. Fig. 45. 



Paris basin. 



Pupa muscorum. 



Helix plebcium. 



Fig. 46. 



Bulimus 

 lubricus. 



Att recent ; and also fossil from Loess of Rhine. 



The Ampullaria (Fig. 46.) is another genus of shells, inhabit- 

 ing rivers and ponds in hot countries. Many 

 fossil species have been referred to this genus, 

 but they have been found chiefly in marine for- 

 mations, and are suspected by some concho- 

 logists to belong to Natica and other marine 

 genera. 



All univalve shells of land and freshwater 

 species have entire mouths ; .and this circum- 

 stance 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 conclude that the formation is marine. The aperture 

 is said to be entire in such shells as the Ampullaria and the land 

 shells figured in this page, when its outline is not interrupted by 

 an indentation or notch'such as that in Ancillaria (Fig. 48.) ; or 

 is not prolonged into a canal, as that seen at a in Pleuromota 

 (Fig. 47.). 



The mouths of a large proportion of the marine univalves 



from the Jumna, 



