THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 181 



another paper ^ I have shown that two of these halolimnic 

 genera^ Paramellania and Nassopsis, are conchological'.y 

 indistinguishable from the genus Purpurina of the old 

 Jurassic seas; and although the genus Melanopsis does not 

 come at all sufficiently near Nassopsis to be placed within it, 

 I think there is some evidence for regarding this Tertiary 

 genus as having arisen as a modification of the old Purpurine 

 stock which died out in the sea during the later Secondary 

 formations, but which it is possible still lives in that part of 

 the old Jurassic seas to which Lake Tanganyika appears to 

 correspond. 



As I have shown elsewhere, the most remarkable feature 

 about the genus Nassopsis is the very curious way in which 

 the nervous systems of the forms belonging to it foreshadow 

 those of the terrestrial Cyclophoridse. Hitherto no satisfactory 

 explanation has been given as to the origin of the Cyclophoran 

 nervous system ; and it is in the highest degree probable, from 

 the close similarity which it bears to that of the Jurassic 

 Nassopsis, that the Cyclophoridae originated from some true 

 fresh-water derivative of the Purpurinas, such as the genus 

 Pyrgulifera, which is found in the fresh-water deposits of 

 the chalk. In the table (PI. 19) I have represented the 

 Purpurina of Tanganyika as the direct continuation of the 

 similar forms found in the old Jurassic seas; while it is 

 suggested that the more modified Pyrguliferas of the chalk 

 may represent a true fresh-water non-halolimnic development 

 of these forms. 



It will, I think, be obvious that the observations here collected 

 in no way finally dispose of the interesting problem of the 

 origin of the heterogeneous constituents composing the Mela- 

 uiidie as the family at present stands ; but the splitting of this 

 group into three broad divisions, based on the anatomical 

 characters of such of them as have hitherto been investigated, 

 is unquestionably in accord with the morphological facts of 

 the case. What is immediately required is a more detailed 



> ' Quart. Journ. Micr. Sci.,' Vul. U, \)[). 3U— 317, 1898. 



