JURASSIC FRESHWATER SHELLS 199 



The Cerithiidse are certainly not older than the Trias ; 

 this, then, is also the earliest date we can assign to the 

 Melaniidse, which may certainly have originated in 

 portions of the Trias sea, isolated from the open ocean 

 and converted by the influx of rivers into lakes. 



Neritina is in very much the same case as Melania, 

 except that it is still an inhabitant of our area. It has 

 affinities with Trochus and Turbo, the representatives of 

 families which were already in existence in Cambrian 

 times, but it is still more directly connected with the 

 Neritidse, marine forms, many of which are not averse to 

 brackish or even fresh water ; 

 they are not known before the 

 Trias, and this, then, is the 

 limit to the past history of 

 Neritina. 



A similar argument may be 

 applied to Hydrobia, a brackish 

 water shell, first cousin to our 

 existing Bythinia ; its ancestral 

 stem is the family of Littorinids, 

 of which a familiar representa- 

 tive is the common periwinkle. 

 They date from the Silurian, but 

 Hydrobia is probably connected with the main stem 

 through the intermediary family of the Eissoids, which 

 make their first appearance at the beginning of the 

 Jarassic period. 



Paludina traces its descent from the ancient stem of 

 the Trochids and Turbonids, which, as already mentioned, 

 are found fossil in Cambrian sediments. It may possibly 

 have originated in Devonian lakes, but its absence from 

 Carboniferous and Permian rocks would suggest a later 

 date. 



Valvata (Fig. 69) is a little shell of considerable interest. 



FIG. 69. Valvata piscinalis, 

 Mull. The gill plume, an 

 archaic feature, is well 

 displayed. Magnified. 

 From Bronn's " Thier- 

 reichs." 



