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 



IAmnotrochus thomsoni, upper, compared with Littorina sulcata, 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 



