Ch. III.] 



FROM MARINE FORMATIONS. 



29 



GrypTima incurva, Sow. ((?. 



arcuata. Lam.) upper 



valve. Lias. 



ludina. 



(See figures.) 



Fig. 31. 



Lamarck divided the bivalve mollusca into the 

 Dimyary, or those having two large muscular 

 impressions in each valve, as a b 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, Planorbis, Lymnea, and Pa- 

 But to these are occasionally added Physa, Sue- 

 Fig. 32. Fig. 33. 



Planorbis euomphalus ; 

 fossa Isle of Wight 



Lymnea longiscata ; 

 fossil. Hants. 



Pdludina lenta ; 

 fossil. Hants. 



tinea, Ancylus, Valvata, Melanopsis, Melania, and JVeritina. (See figures.) 



Fig. 34 Fig. 35. Fig. 36. Fig. 37. 



Succinea amphibia ; 

 fossil. Loess, Ehine. 



Ancylus elegans ; 

 fossiL Hants. 



Valvata ; Plvysa hypnorum ; 

 fossil. recent. 



Grays, Essex. 



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

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



Fig. 33 



Fig. 39. 



Fig. 40. 



Fig. 41. 



Auricula ; 

 recent. Ava. 



Mdanopsis buc- 

 dnoidea ; recent. 

 Asia. 



cept in the animal, 

 thinner* 



The shell, however, of the Ancylus is usually 

 * Gray, Phil. Trans. 1835, p. 302. 



