THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 187 



extraordinary state of asymmetry which is encountered in the 

 same region on the right. On this side the pleural ganglion 

 gives off a nerve (fig. 34, 8) which appears at first sight as if 

 it would form the right pallial anastomosis, either in the 

 region of the subintestinal ganglion or along the course of the 

 right pallial nerve. This nerve, however, after passing directly 

 outwards for some distance bends sharply forwards, branches 

 once, and each of the twigs diminishes rapidly, and dies out in 

 the parietes of the mantle and the body-wall. The great 

 mantle nerve which is given off from the subintestinal gan- 

 glionic trunk (fig. 34, 10) passes outwards, and is also distri- 

 buted, without forming any connection with the pleural 

 branch, to the mantle of the right side. Neither is there any 

 connection between the pleural and subintestinal ganglion 

 beneath the oesophagus. We have thus on the right side a 

 condition of things which is almost unique among all the 

 Streptoneurous Prosobrancliiata which have hitherto been 

 investigated. It is neither zygoneurous nor yet dialyneurous, 

 and the condition of these parts finds its only analogy in the 

 rather unsatisfactory descriptions given by Bouvier ^ of the 

 nervous system of Solarium and the Scalarids. In general ar- 

 rangement, and apart from the above singular feature, the 

 character and arrangement of the cerebral, pleural, and intes- 

 tinal ganglia with their nerves and connections show a marked 

 and indisputable similarity to those of the corresponding parts 

 in such forms as Strombus, Pterocera, Cancellaria, 

 Voluta, and their associates. The wide distribution of the 

 ganglionic cells in the nerve-cords of Typhobia is a remark- 

 able and undoubtedly primitive feature ; while the fact that 

 the nervous system of Typhobia foreshadows and is similar 

 to those of several rather widely separated modern marine 

 genera is direct and incontrovertible evidence, so far as it goes, 

 that these molluscs are old and modified marine forms. 



The pedal ganglia project forwards and are curiously ex- 

 tended in front by two colossal nerves (fig. 35, 13), which at 

 their points of origin possess ladder-like connections one with 

 ' ' Ann. des Sci. Nat.,' iii, i^ 1887, pp. 156—167. 



