Ch. Ill] 



FROM MARINE FOEMATIONS. 



29 



Gryphcea incurva, Sow. {G. 



arcuata, Lam.) upper 



valve. Lias. 



ludina. (See figures.) 



Fig. 31. 



Lamarck divided the bivalve mollusca into the 

 Dlmyary, or those having two large muscular 

 imj^ressions in each valve, as a 6 in the Cyclas, 

 fig. 25, and the Monomyary, such as the oyster 

 and scallop, in which there is only one of these 

 impressions, as is seen in fig. 30. Now, as none 

 of these last, or the unimuscular bivalves, are 

 freshwater, we may at once presume a deposit in 

 which we find any of them to be marine. 



The univalve shells most characteristic of fresh- 

 water deposits are, Planorhis, Lymnea, and Pa- 

 But to these are occasionally added Physa^ Sue- 



Fiff. 32. Fig. 33. 



PlanorMs euomphalus ; 

 fossil. Isle of Wight. 



Lymnea longiscata j 

 fossil. Hants. 



Paludina lenta , 

 fossil. Hants. 



cinea, Ancylus^ Valvata, Melanopsis, Melania^ and Neritina. (See figures.) 



Fig. 34. Fig. 35. Fig. 36. Fig. 87. 



Suecinea amphibia ; 

 fossil. Loess, Ehiue. 



Aneylus elegans ; 

 fossil. Hants. 



Valvata ; 



fossil. 



Grays, Essex. 



Physa hypnorum ; 

 recent. 



In regard to one of these, the Aneylus (fig. 35), Mr. Gray observes 

 that it sometimes difters in no respect from the marine Siphonaria, ex- 

 Fig. 83 Fig. 39. Fig. 40. Fig. 41. 



Auricula ; 



Melania 



Physa eolum- 



3felanopsis luc- 



recent. Ava. 



inquinata. 



naris. Paris 



cinoidea; recent. 





Paris basin. 



basin. 



Asia. 



cept in the animal. The shell, however, of the Ancyhcs is usually 

 thinner.* 



* Gray, Phil. Trans. 1885, p. 302. 



