1 6 THE TANGANYIKA PROBLEM. 



a very striking fact, and it is all the more interesting when we 

 reflect that the characters possessed by the types of the 

 primary fresh-water series are those which there is very good 

 reason for believing were once possessed by some of the 

 members of the normal oceanic fauna very long ago. Thus 

 among the mollusca which we have just relegated to the 

 primary fresh-water series we find that the Prosobranch 

 genus Vivipara is with reason regarded by students of com- 

 parative anatomy as possessing those particular structural 

 characteristics which must have marked the transition of the 

 old marine diatocardiate forms into the later Tsenioglossa. 

 Melania also is a form which has an extremely simple Ceri- 

 thoid organisation, connecting up the Ceritho-Litterinoids 

 with the Strombus and Natica groups. Limnea, which is 

 found in almost every fresh water throughout the globe, 

 is a form belonging to a series of gastropods which 

 probably anteceded the modern marine Opisthobranchs. 

 We are confronted with similar facts respecting the 

 structural relationships of all the other molluscs belong- 

 ing to the primary fresh- water series. It is, however, 

 not only with respect to the mollusca that we find the^ 

 peculiarities among the constituents of the primary fres - 

 water series : the same phenomena are encountered in re- 

 gard to the Crustacea, and markedly among the fresh-water 

 fishes such as the Australian, African, and American ganoids 

 and the like. From these considerations it would appear to 

 be suggested that the primary fresh- water stock belongs to 

 an ancient type of fauna which there is reason to believe was 

 once widespread in the sea, but which for some reason 

 wholly unapparent on the face of things has latterly become 

 restricted to the fresh waters of the globe. 



If, however, we examine the palaeontological evidence 

 which exists respecting the first appearance of the types 



