466 ME. J. E. S. MOORE ON THE MOLLUSCS [Juiie 18, 



water. It is, however, in its organization, as I have ah'eady 

 shown, an extremely primitive Tajnioglossan, and forms in itself 

 one of the few Architaenioglossates of which we have any 

 knowledge. Erom anatomical considerations alone it would be 

 extremely difficult to discover the nearest allies which Nassojysis 

 has or did po.:isess ; and the only clue we have to its past ancestry 

 is the highly remarkable and interesting fact that the numerous 

 varieties of its shell, for which the genus is famous, correspoaid 

 in a specific sense with the different species of Purpurina of the 

 old Jurassic seas. _From the pritnitive anatomical characters of 

 Nassojjsis and from the correspondence of its shell with those 

 ofc' the Purpurinas, it would appear probable that this mollusc is 

 the sole survivor of a primitive and extinct marine stock. 



Lastly, we have to consider that pecidiar mollusc from Tan- 

 ganyika wdiich Smith described under the name of Neotlinuma. 

 Is Ncotlmuma (PI. XXV. figs. 1, 4, & PI. XXVI. figs. 1, 3, 4, 5, 6) 

 a Vivipara or not? When first examined, this mollusc would 

 certainly be taken for a Vivipara^ and there is no question 

 that it is a close ally of those Viviparas with which we are 

 anatomically acquainted, but it differs in one y(^yy remarkable 

 feature from all the Viviparas which have hitherto been examined. 

 It may be remembered that Bouvier has described the Viviparas 

 as being characteristically dyaloneurous in the arrangement of 

 their nervous system (PI. XXV. fig. 5) — that is, the pallial nerve, 

 which springs from the supra- and sub-intestinal ganglia, unites 

 with a branch euianating from the right and left pleural ganglia ; 

 and it may be remembered that the deviations that Bouvier noticed 

 from this type of arrangement were constituted in different 

 molluscs by a direct union of the nerve springing from the right 

 pleural ganglion not with the pallial nerve in the body-wall, but 

 with the sub-intestinal ganglion itself. This zygoneurous deviation 

 from the dialoneurous condition of things in the ordinary Fiviji)ara 

 has hitherto, invariably I believe, been found either only on the 

 right side or on both sides of the nervous system ; and it is therefore 

 extremely interesting to note that Neotliauma presents us with a 

 Vivipara possessing a single zygoneurous connection on the left 

 and the normal dialoneurous relationship of the nerves upon the 

 right (compare PI. XXV. figs. 4 & 5). 



So much for the affinities of the halolimnic molluscs as proclaimed 

 by their anatomy. It will, I think, be readily conceded that, after 

 what has been said, it is difficult, or, I may say, impossible, to 

 view these extraordinary molluscs as either the forerunners or the 

 derivatives of the freshwater molluscs which we find in the lakes 

 and rivers all over the world today. They are, however, readily 

 intelligible if we regard them as the forerunners of several marine 

 gi'oups, such as the Strombidse, the Naticas, and the early Ciriths, 

 to which I have referred. They have the organization which we 

 should ascribe to the ancestors of these groups, and their ancestors 

 lived some time or other in the sea before their present representa- 

 tives took shape. So far, then, as the first line of investigation is 



