344 
THE TANGANYIKA PROBLEM. 
words, they still retain the characters of a fauna belonging 
to some departed age. This being so, it also becomes 
apparent that it is at any rate possible that some of the 
hard parts of these halolimnic animals, the shells of the 
gastropods for example, the spicules of the sponges, and so 
on and so forth, may present the structural peculiarities 
which typify the same kind of organisms belonging to some 
particular palaeontological epoch. As a matter of fact, even 
before I was assured of the really primitive character of the 
halolimnic invertebrates, I had become convinced, while still 
upon the shores of Tanganyika, that the strange shells 
peculiar to the region were very similar to some other shells 
either living or extinct which I had already seen elsewhere, 
and on searching through the conchological representatives 
of the different geological eras, it was found that the very 
remarkable facies which the shells of the halolimnic gas- 
tropods possess, is unmistakably again presented by a 
similar number of gastropods, which are characteristic of 
the deposits left by the old Jurassic seas. In following up 
this line of investigation we have been greatly indebted to 
Mr. Hudleston, who allowed me to examine and have 
drawings made of the suitable specimens contained in his 
unique collection of Jurassic forms. From this collection, 
and from that contained in the British Museum, it was 
possible to find types which correspond often in a specific 
sense with the shells of the living halolimnic group. And 
in order that the reader may fully appreciate the nature of 
this comparison, I have reviewed in sequence the different 
corresponding types. 
Beginning with the genus Paramelania from Tan- 
ganyika, we find that, among the numerous fossil remains 
of gastropods in the marine Jurassic deposits, there are a 
number of allied forms to which the generic name of 
