THE TANGANYIKA PROBLEM. 271 



and also of a number of divergent forms, such as Natica, 

 Capulus, Cancellaria and StrutJiiolaria. In minor details 

 the nervous system of the TypJwbia group approaches more 

 closely to that of Capulus and Cancellaria than it does to 

 that of either Cerithium or M. amarula, while in the 

 curious features of their gastric apparatus all the members 

 of the TypJwbia group are really closely approximated to 

 such forms as Strombus and Pteroceras and Aporrhais. 



The members of the TypJwbia group thus present a 

 type of organisation which appears to stand at the parting 

 of the ancestral ways of such widely divergent marine 

 groups as those characterised by the following genera : 

 Strombus, Pteroceras, Cerithium, Strut Jiiolaria, Cancellaria, 

 AporrJiais and Capulus ; for each of these diverse, marine, 

 modern gastropods contain, underlying their own peculiar 

 specialisation, more or fewer of the characters which are 

 compounded in the members of the TypJwbia group. 



A similar and still more interesting result is obtained 

 from the comparative study of the two genera CJiytra and 

 LiinnotrocJius. The whole anatomy of CJiytra is singularly 

 like that of Capulus. We have in both forms the same 

 arrangements of the nerves, the pallial connectives being 

 identical in both genera. So also the radula of CJiytra is 

 curiously like that of Capulus, and it corresponds even in 

 minor details with the ally of CJiytra, Hippnyx. CJiytra, 

 however, differs from both these genera, and all their 

 more remote naticoid allies in the possession of a style 

 sac. And what is even more remarkable, in the reten- 

 tion in its stomach of a well-developed spiral caecum, 

 a structure which, so far as I know, is only present 

 in Nassopsis elsewhere among the Taenioglossa, but 

 which, as is well known, is a characteristic feature of 

 the stomachs of the more primitive Rhipidoglossa, such 



