THE TANGANYIKA PROBLEM. 277 



a labial commissure, which, as Bouvier has shown, is a 

 special feature of the Rhipidoglossa and their allies. But, 

 not only is Nassopsis an archi-tsenioglossan in the character 

 of its nerves, we find, as I have shown above, that this 

 type has a most unique and primitive gastric apparatus ; 

 which, like Chytra, unites at the same time the presence of 

 a style sac, and a style with the spiral stomachic caecum 

 common to the Rhipidoglossa. Nassopsis is thus, on 

 anatomical examination, shown clearly to be a new type of 

 archi-taenioglossa, and, consequently, a genus which will be 

 of great value in the future study of the relationships of the 

 different sections of the Prosobranchiata. As a matter of 

 fact, iVassopsis, in the Rhipidoglossate character of its 

 stomach, together with the possession of a crystalline style, 

 is a much more striking archi-taenioglossate form, than 

 even Vivipara itself* 



Having followed these matters relating to the affinities of 

 A T assopsis, we have completed the consideration of the 

 structure of the halolimnic gastropods in Tanganyika ; and 

 so far as our knowledge of the vast hordes of Prosobranchs 

 which live in the sea and the fresh-water of the earth at the 

 present time make such an attempt possible, we have esti- 

 mated the affinities of the different individual members of 

 the halolimnic group. None of these molluscs, as we have 

 seen, appear capable of being regarded as in any way 

 related to the fresh-water molluscs which exist elsewhere in 

 the world to-day ; while the whole group presents us with 

 an array of characters which at once carries them back 

 to the position of ancestors of the numerous forms, chiefly 

 marine, which their structure appears distinctly to fore- 



* For the reasons stated in the last chapter, I have not included the genera Neothaumus 

 and Stanleya in the halolimnic group, while for Syrnolopsis we have as yet not sufficient 

 material for a complete investigation. 



