200 FRESHWATER FAUNA 



Its nervous system almost precisely resembles that of 

 Bithynia, and it has close affinities with the Kissoids ; 

 but it is distinguished by more than one strikingly 

 archaic character, of which the possession of a bipecti- 

 nate gill plume is the best known ; it is also said to 

 be hermaphrodite, but Bouvier considers that this needs 

 confirmation. The Eissoids, as we have seen, are derived 

 from the Littorinids, and were, no doubt, once connected 

 by a series of intermediate forms. These may be spoken 

 of as pro-Eissoids, and there would be nothing unreason- 

 able in the supposition that Valvata is the transformed 

 descendant of one of these pro-Eissoid links, and retains 

 some of the archaic characters of its ancestors. The 

 transformation might well have been accomplished in 

 some Trias lake. 



The affinities of Cyrena and Corbula also indicate the 

 Trias as the earliest date at which they are likely to have 

 come into existence. 



Thus the oldest known horizon on which Mesozoic 

 freshwater fossils occur in a favourable state of preserva- 

 tion is the inferior Oolite, and a consideration of the 

 zoological affinities of the various genera which this has 

 yielded leads to the conclusion that their origin cannot 

 be pushed further back than the Trias, though it may 

 possibly be later ; while the known occurrence of lakes 

 during the Trias points to the existence of circumstances 

 favourable to the evolution of freshwater forms of life. 



Passing upwards in the stratified series we reach the 

 deposits of the Purbeck and Weald, which at their base 

 show a gradual passage from marine to freshwater con- 

 ditions, but we cannot definitely claim them as lacustrine ; 

 this they may be, but according to the prevailing opinion 

 they are to be regarded as deltaic sediments. They 

 present us with most of our existing freshwater mollusca, 

 and with many genera that no longer inhabit northern 



