28 



DISTINCTION OF FKESHWATER 



[Cn. III. 



Altlioiigli freshwater formations are often of great thickness, yet they 

 are usually very limited in area when comj)ared to marine deposits, 

 just as lakes and estuaries are of small dimensions in comparison with 

 seas. 



We may distinguish a freshwater formation, first, by the absence of 

 many fossils almost invariably met with in marine strata. For example, 

 there are no sea-urchins, no corals, and scarcely any zoophytes ; no 

 chambered shells, such as the nautilus, nor microscopic Foraminifera. 

 But it is chiefly by attending to the forms of the mollusca fliat we are 

 guided in determining the point in question. In a freshwater deposit, 

 the number of individual shells is often as great, if not greater, than in 

 a marine stratum ; but there is a smaller variety of species and genera. 

 This might be anticipated from the fact that the genera and species of 

 recent freshwater and land shells are few when contrasted with the ma- 

 rine. Thus, the genera of true mollusca according to Blainville's system, 

 excluding those of extinct species and those without shells, amount to 

 about 200 in number, of which the terrestrial and freshwater genera 

 scarcely form more than a sixth.^' 



Almost all bivalve shells, or those of acephalous mollusca, are marine, 



Fiff. 25. 



Cyclas ohovata ; fossil. Hants.. 



Cyrena consol)rina ; fossil. Grays, Essex. 



about ten only out of ninety genera being freshwater. Among these 

 last, the four most common forms, both recent and fossil, are Cyclas^ Cy- 



Fiff. 27. 



Fi?. 2S. 



Fig. 29. 



Anodonta Cordierii ; Anodoyitalatimarginatiis ; TTnio UUoralis ; 



fossil. Paris. recent. Bah'ia. recent. Auvergne. 



rena, Unio, and Anodonta (see figures) ; the two first and two last of 

 which are so nearly allied as to pass into each other. 



* See Synoptic Table in Blainville's Malacologie. 



