Cn. III.] 



FROM MARINE FORMATIONS. 



29 



Lamarck divided the bivalve rnolmsca into 

 the Dimyary, or those having two large mus- 

 cular 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 pre- 

 sume a deposit in which we find any of them 



to be marine. Gryphma incurva, Sow. (£. 



The univalve shells most characteristic of arcuata, Lam.) upper valve. 



Lias. 



fresh-water deposits are, Planorbis, Limnoea, 



and Paludina. (See figures.) But to these are occasionally added 



Fig. 31. 



Fig. 32. 



Fig. 



Planorbis euomphalus; 

 fossil. Isle of Wight. 



Limnma longiscata ; 

 fossil. Hants. 



Paludina lenta ; 

 fossil. Hants. 



Physa, Succinea, Ancylus, Valvata, Melanopsis, Melania, Potamides, 

 and Neritina. (See figures.) 



Fie. 34. 



Fig. 35. 



Fig. 36. 



Fig. 37. 



Succinea amphibia ; 

 fossil. Loess. Ehine. 



Fig.: 



Auricula ; 

 recent. Ava. 



Ancylus elegans ; 

 fossil. Hants. 



Fig. 39. 



Fiff. 40. 



Yalvaia ; Physa hypnorum , 

 fossil. recent. 



Grays, Essex. 



Fig. 41. 



Melania Physa colum- 



inquinata. naris. Paris 

 Paris basin. basin. 



Melanopsis buc- 



cinoidea ; recent. 



Asia. 



* The freshwater Mulleria, which has two muscular impressions when young, has 

 only one in the adult state, thus forming a single exception to the rule. 



