THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 159 



below the gill. It is a simple ropy structure at its base^ but it 

 becomes very distinctly pectinated at its distal extremity (PI. 

 14, figs. 2, 14, OS.). 



The nervous system of T. rufofilosa (PI. 14, fig. 6) con- 

 sists of two large cerebral ganglia, closely united together as 

 in the genus Melania (PL 15, fig. 58). 



The cerebral ganglia are indistinguishably fused with the 

 pleural ganglia below, but from their respective loci there 

 spring the super- and sub-intestinal cords (PI. 15, fig. 6), which 

 pass to the super- and sub-intestinal ganglia. Prom the locus 

 of the left pleural ganglion a nerve passes out towards the 

 mantle, and this nerve apparently anastomoses with a branch 

 from the subintestinal ganglion (PI. 15, fig. 6). The nervous 

 system is therefore dialyneurous on the left. 



In like manner there springs a nerve from the right pleural 

 ganglion, which also unites with a branch from the super- 

 intestinal. The nervous system is therefore entirely dialy- 

 neurous on both sides, and when viewed from above it is 

 practically indistinguishable from the similar view of the 

 nervous system of Melania (?) which I have given in PI. 15, 

 fig. 5. On the under side of the cerebral ganglion, however, 

 there is a curious and, so far as I am aware, unique median 

 protuberance (PI. 14, fig. 8, m.p.c.g.), but which is in itself 

 quite sufficient to distinguish the nervous system of T. rufo- 

 •filosa from that of any known Melania. The cerebro- and 

 pleuro-pedal commissures are of median length, and unite at 

 the upper extremity of the well-developed pedal ganglia (PI. 14, 

 fig. 8, j9. g.). On the upper posterior faces of the pedal ganglia 

 there are slight and peculiar projections (PL 14, fig. 8). 



The otocysts are fairly large; they lie behind and slightly 

 above the pedal ganglia, the otocyst nerve passing diagonally 

 across the outer faces of the cerebro- and pleuro-pedal con- 

 nectives to the cerebral ganglia on each side (PL 14, fig. 8, 

 ot.). The otoliths are rectangular, small and numerous; 

 there are a small osphradial and two visceral ganglia. 



VOL. 42, PART 2. NEW SERIES. 



