THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 195 



The nervous system in Bythoceras is very interesting, 

 since it is absolutely unlike that possessed by the genus 

 Nassopsis, and closely simulates the type described by 

 Bouvier ^ as typical of the genus Cerithiura. It also strongly 

 resembles that of the genus Tanganyikia. Viewed from 

 above (PI. 21, fig. 6), the cerebral ganglia are seen to be 

 closely fused together, while the left pleural and subintestinal 

 ganglion, as in Cerithiura, form a single massive trunk, 

 which at its hinder extremity gives rise to the subintestinal 

 and visceral nerve-cords, and to the right pallial nerve (fig. 6). 

 From the right pleural ganglion a nerve passes out to the 

 mantle, and a branch from this anastomoses with a branch on 

 the pallial nerve just described. In like manner on the left 

 the pleural ganglion gives birth to a nerve on that side (fig. 6), 

 which passes out and probably anastomoses with a twig given 

 off from the supra-intestinal ganglion, but I was not able to 

 trace this nerve throughout its entire course. 



Unlike the subintestinal ganglion, the supra-intestinal gan- 

 glion is carried on a very long supra-intestinal connective 

 (PI. 21, tig. 6, sup. int. g.) exactly as it is in Cerithium 

 or Aphorais. Viewed from the side (PI. 21, fig. 10) the 

 cerebro-pedal and pleuro-pedal connectives are seen to be of 

 considerable length, rather longer than the same structures 

 in Voluta, but not so long as those in Nassopsis or in 

 Strombus. The pedal ganglion has the usual bulbous form 

 as in the true Cerithidae, and in like manner there pass from 

 the lower extremity of each pedal ganglion two predominant 

 foot nerves (fig. 10). 



The otocysts lie behind the pedal ganglia, and the otocyst 

 nerves pass directly between the cerebro-pedal and pleuro- 

 pedal connectives to the cerebral ganglia on each side (PI. 21, 

 fig. 10). The otocysts are not large, and are round, as dis- 

 tinguished from those of Nassopsis. The otoliths are small, 

 rectangular or barrel-shaped, and numerous. 



The reproductive apparatus is very simple, and in both 

 sexes consists of a genital gland which occupies the upper 

 1 'Ann. Dis. Sci. Nat.,' 1887, pp. 131—155, pi. vii, aud figs. 



