198 FRESHWATER FAUNA 



succeeding beds of the lower Lias we find Cyrena, 

 Neritina, and, according to C. Moore, a species of 

 Planorbis and tw r o species of Valvata. Moore's identifica- 

 tion of the last-named genus has been disputed, but I see 

 no reason to doubt the accuracy of this cautious and 

 sagacious observer. It is scarcely necessary, however, to 

 argue the point, since recently Valvata has been described 

 by M. Cossman * from the inferior Oolite of St. Gauthier, 

 in the department of Indre, France. In the inferior 

 Oolite we also find Cyrena, Corbula, Planorbis, Paludina, 

 Hydrobia, Neritina, and Melania. 



Thus even in these early Jurassic times a large 

 number of our freshwater shells were already in 

 existence, and this is precisely what their cosmopolitan 

 distribution might have led us to expect. No doubt their 

 origin antedates their first appearance, by how long an 

 interval it is difficult to say, but in some cases a limit 

 may be found by reference to the marine families with 

 which they are most closely allied, and from which they 

 have probably been derived. This is rendered possible by 

 the fortunate fact that marine are much more abun- 

 dant than freshwater shells, and far more frequently met 

 with in a fossil state. 



Melania, a member of a very large and heterogeneous 

 family, was for a long time a common inhabitant of our 

 rivers and lakes ; but, with the increasing refrigeration of 

 our climate, it left us for warmer regions, and is not 

 now met with nearer than the south of Europe. The 

 family Melaniidae are freshwater representatives of the 

 Cerithiidae, which also affect the warmer zones, and 

 are chiefly distinguished from the Melaniidae by their 

 marine habit. In connexion with this it may be observed 

 that the majority of the Melaniidse are viviparous ; this is 

 not the case with the Cerithiidse. 



* Bull. Soc. Geol de France, 1899, Ser. III., vol. xxvii. p. 136. 



