THR MOLLUSCS OF THE GREAT AFRICAN LAKES. 177 



does not incline to either of these extreme types, but. like 

 Lamellaria (fig. 3), it represents a third modification, due to 

 the simultaneous shortening of both the supra- and sub-intes- 

 tinal cords. It would appear, therefore, that S. z on at a approxi- 

 mates, both in its general anatomy and in the minutise of its 

 nervous system, to those Naticoid forms which have been 

 already examined. 



Unquestionably, if judged by the nervous system alone, it 

 would be placed, not only near, but within the genus Lamel- 

 laria; but, sincte it differs from this genus in its external form, 

 and from all other Naticoid genera in the characters of its 

 radular dentition, I think it will be well to keep it in a genus 

 by itself, which is to be regarded as closely related to, but 

 distinct from, Lamellaria. Unquestionably Spekia can 

 no longer be regarded as having any relation whatever to the 

 members of the family to which the genus Lithoglyphus 

 belongs, and with which S. zonata was unhesitatingly placed 

 by the conchologists. We have here, therefore, obviously one 

 more example of the impossibility of making correct determi- 

 nations from the shell structure alone, while at the same time 

 it forms an equally striking instance of the marine nature of 

 the halolimnic fauna of the lake in which it lives. 



Turning now to the question of the affinities of the genus 

 Tanganyikia, it will be seen that the nerves, no less than the 

 general anatomy of this genus as represented by T. rufo filosa, 

 correspond to those of a number of well-known molluscous 

 forms. I showed in the descriptive part of this paper that, but 

 for the ventro-median protuberance on the cerebral ganglia, 

 the nervous system of rufofilosa corresponds almost exactly 

 to that of Melania amarula, as described by Bouvier (loc. 

 cit.), and with that of the closely allied species of Melania 

 represented in PI. 15, fig. 5. There is nothing in the general 

 anatomy, or in the reduced radula of T. rufofilosa, which need 

 lead us to suppose that it is not really closely related to both 

 these types. The only marked anatomical difference presented 

 lies in the possession by T. rufofilosa of the curious accessory 

 reproductive apparatus which I have described. But I have 



