THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 161 



Serpula respectively. The family Limnseidse, which is 

 now so universal in its distribution^ does not certainly extend 

 further back than the Jurassic period. The same is true 

 of the fresh-water Melaniidse and of the Paludinidse, but 

 I need scarcely point out that it is necessary to use the 

 greatest caution in drawing any inferences respecting the 

 date of origin of the true fi'esh-water forms from these apparent 

 facts. It may, however, be taken as approximating to the 

 truth to say that although the typical and universal fresh-water 

 molluscs of the present do not appear upon the stage of life as 

 such before the Jurassic period, they almost certainly origi- 

 nated from a series of marine types which had become com- 

 pletely differentiated from their oceanic associates long before 

 this time. Some of these antecedent organisms are probably 

 represented in the palseontological record by those extinct 

 genera with which the earliest known modern fresh-water 

 types are usually associated. ^ 



The facts of morphology are themselves in harmony with 

 such a view, for in their anatomy the living fresh-water 

 molluscs do not approximate to any of the more modern 

 marine genera; they hark back to those more permanent 

 marine types which were in existence long before Jurassic 

 times. They certainly bear no resemblance to the generalised 

 conceptions or archetypes of the more modern marine genera 

 which appeared during Tertiary and post-Tertiary times, such 

 as Strombus, Pteroceros, Rostellaria, Conus, Mitra, 

 Chenopus, and the like. It is this fact that is of first 

 importance to us here ; for if it should be found that in 

 some district at the present time there exists a fresh-water 

 fauna which departs from the normal and universal type in 

 the possession of genera which approximate to those that are 

 undoubtedly modern and marine, we shall have very strong 

 prima facie evidence for regarding these organisms as recent 



1 II; is quite possible that many of the old so-called fresh-water deposits are 



in reality marine, since the forms which became exclusively fresh water as 



time went on probably made their appearance in the sea first, as so many of 



the more recently derived fresh-water types have done, — prawns, for example. 



VOL. 41, PART 1. NEW SERIES. L 



