THE TANGANYIKA PROBLEM. 
2 5 r 
obtained by Smith, Guatkin referred it to the types 
approximating to the genus Cerithium , while Smith 
himself remarked upon its similarity to the radula of the 
Planaxidae. It also corresponds very closely to the radulae 
Fig. 34. — Semi diagram of the anatomy of Nassopsts nassa. The 
mantle has been cut open from above, and the nervous system is 
seen from above in black. Opposite sin., the gill ; cryst. s., 
crystalline style ; sp. c., spiral caecum; si., stomach. 
of several Littorinas. The oesophagus is long, narrow, 
and simple, and leads into a large stomachic chamber, on 
the walls of which there are numerous glandular folds, 
and a very curious and striking spiral caecum on the 
