i6 
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 Taenioglossa. 
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 these 
peculiarities among the constituents of the primary fresh- 
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 
