28 GILBERT C. BOURNE. 



to the thick anterior commissure connecting the dorsal 

 portions of the ganglia, there is a single anterior thin com- 

 missure connecting the ventral portions, but this is the only 

 trace of the usually numerous cross commissures of other 

 lowly organised Gastropoda. Such a concentration of the 

 pedal centres is very unusual if not unique among Aspido- 

 branchia, and indicates that Incisura, and, if one may judge 

 from the similar relations indicated in Pelseneer's figures of 

 S. costata, the Scissurellidae in general are highly specialised. 

 Much has been written about the significance of the dorsal 

 and ventral moieties of the pedal cords of archaic Gastropods. 

 The French authors hold that the upper moiety is pleural, or, 

 as they say, pallial, the lower moiety pedal in character. 

 Pelseneer and most English and German authors hold that 

 both moieties represent pedal centres. The facts in Incisura 

 seem to uphold the latter view. I have no wish to i-e-enter 

 upon a controversy which has become almost wearisome by 

 repetition, but may state that in Incisura the cerebro-pedal 

 connectives certainly join the dorsal moieties of the ganglia; 

 that the epipodial nerves are certainly given off from the 

 dorsal moieties, and that whereas the left symmetrical pallial 

 nerve is undoubtedly given off from the left pleural ganglion, 

 the right symmetrical pallial nerve certainly appears to be 

 given off from the dorsal moiety of the right pedal ganglion 

 and not from the right pleural, both in Incisura and Fissurella. 

 Advocates of the French view will take this last fact as 

 evidence in support of their theory. The nervous system of 

 Incisura certainly bears no resemblance to that of Pleuro- 

 tomaria. On the whole it most nearly resembles that of the 

 Fissurellidte, in which family the pedal cords, though still 

 elongate and ganglionic, and provided with several cross- 

 commissures, have undergone a considerable reduction in 

 length as with those of other Rhipidoglossa. 



Thesense organs. — The eyes, as already stated, are closed 

 and provided Avith a distinct lens. Their structure resembles 

 that of the eyes of the Fissurellidte, and differs from the eyes 

 of the Pleurotomariidfe and Trochidas, which are open. 



