THE TANGANYIKA PROBLEM. 
349 
living and the dead forms correspond. Not only do the 
above halolimnic forms find their almost exact Jurassic 
parallels, it appears also that the remarkable shell of the 
Tanganyika genus, Chytra, is repeated among the varieties 
of the Jurassic genus Onustus ( = Xenophera ). Again we 
find that the shells of the Tanganyika genus, Spekia 
Limnotrochus thomsoni, upper, compared with Littorina sidca/a, lower. 
(p. 351), are practically indistinguishable from the fossil re- 
mains of the shells of the marine Jurassic genus Neridomus 
represented on the same page. Nor does the comparison 
end here. There is among the gastropods of the halolimnic 
group a very remarkable and characteristic shell which 
Smith named Melania admirabilis. It is a Cerithoid form 
totally unlike any other living type which is known, but 
