44 



LYELL'S ELEMENTS OF GEOLOGY. 



Distinction of Freshwater from Marine Formations. 



the terrestrial and freshwater genera scarcely form more than a 

 sixth. 



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

 marine, about ten only out of ninety genera being freshwater. 

 Among these last, the four most common forms, both recent and 

 fossil, are Cyclas, Cyrena, Unio, and Anodonta (see figures ;) 

 the two first and two last of which are so nearly allied as to pass 

 into each other. 



Fig. 23. 



Fig. 24. 



Cyclas obovata ; fossW. Hants. 



Fig. 25. 



Cyrena trigonula ; fossil. Grays, Essex. 



Fig. 26. Fig. 27. 



Anodonta Cordierii; 

 fossil. Paris. 



Anodonta latimargi- 

 natus; recent. Bahia. 



Unio littoralis ; 

 recent. Auvergne. 



Lamarck divided the bivalve mollusca into the Dimyary, or 

 those having two large muscular impressions in each valve, as 

 ah in the Cyclas, Fig. 23., and the Monomyary, such as the oys- 

 Fig. 28. scallop, in which there is only one 



of these impressions, as is seen in Fig. 28. 

 Now, as none of these last, or the unimus- 

 cular bivalves, are freshwater, we may at 

 once presume a deposit in which we find 

 any of them to be marine. 



The univalve shells most characteristio. 

 of freshwater deposits are, Planorbis, Lim- 

 nea, and Paludina. (See figures.) But to 

 Ostrea vcsicuiaris {Gryphtea these are occasionally added Physa, Succi- 

 giohosa. Sow.) , chalk. ^^^^ Ancylus, Valvata, Melanopsis, Mela- 

 nia, and Neritina. (See figures.) 



