2"]2 
THE TANGANYIKA PROBLEM. 
as Pleurotomaria and Trochus* From the character of 
the nervous system and the radula of Chytra vve should 
have no difficulty in deciding at once that it comes very 
near the genus Capulus and its modern allies. But since it 
is not superficially modified either in its shell or foot, and 
retains the two very primitive characters in its alimentary 
apparatus which have just been described, and which neither 
the living Capulidae nor their allies are known to possess, it 
is obvious that Chytra is ancestral with respect to them. 
But further, Chytra is possessed of many points of resem- 
blance to Xenophora , and this resemblance is by no means 
confined to the shell. The nervous system of Chytra may 
be said to stand half-way between that of Capulus on the 
one hand and Xenophora on the other. In the same way 
Chytra shows in the character of its foot an incipient stage 
in the modification which reaches its maximum in Strombus 
and Aporrhais and their modern allies. These considera- 
tions indicate that we have in Chytra a form which quite 
satisfactorily connects up two fairly ancient marine groups, 
for it possesses the characters of Capulus and its allies, and 
also those of the Xenophoridae themselves. 
In Limnotrochus we have, as I have pointed out, an ally 
of Chytra , and the same inlerences may be drawn from its 
structure. 
'burning now to the group of gastropods composed by 
the genera Paramelania and Bythoceras and their subordi- 
nate species, we find that all these forms, both in their 
radulae and nerves, present us with features which are dis- 
tinctly similar to those of Cerithium ; but they are at once 
dissociated from the Cerithoid group of Prosobranchs, by 
the peculiarities of their gastric apparatus and their posses- 
* It has since been found in a reduced condition in Tttrritella communis. See paper 
by W. B. Randles, “ Anat. Anz.” 1902 ; Bd. xxi., p. 201. 
