INCISUEA (sCISSUBELLa) LYT'J'ELTONENSIS. 31 



the pallial complex into a median position in the roof of the 

 mantle-cavity are all points in which the ScissurellidfB differ 

 from Pleui-otomaria. 



(6) In Pleurotomaria the right kidney lias a distinct duct, 

 with thickened glandular walls in the female ; in the Scis- 

 surellidfB there is no such duct. 



(7) There is no spiral cascum to the stomach in the Scis- 

 surellidse, and the form of the stomach differs largely from 

 that of Pleurotomaria. 



(8) The hepatic orifices are numerous in Scissurellidge, 

 whereas there is only a single orifice in Pleurotomaria. 



(9) The nervous system of the Scissurellidse differs in detail 

 in almost every point from that of Pleurotomaria, particularly 

 in the concentration of the cerebral ganglia; the extreme fine- 

 ness of the labial commissure; the presence of distinct pleural 

 ganglia; the well- developed symmetrical pallial nerves estab- 

 lishing a right and left dialyneury; the presence of distinct 

 supra- and sub-intestinal ganglia; the shortness of the visceral 

 loop; the concentration and abbreviation of the pedal centres. 



Not only are the Scissui'ellidas distinct from the Pleuro- 

 tomariidae, but they are clearly less closely related to them 

 than the Haliotidse or even than the Trochidje and Turbonidae, 

 for the last-named families, though they have lost the labral 

 incision in the shell, as also the right ctenidium and the 

 structures correlated to it, have retained many anatomical 

 features which find their counterpart in Pleurotomaria. 



Where, then, shall we find the nearest relatives of the 

 Scissurellidae ? Though Mr. Hedley was clearly in error in 

 removing lucisura from the Scissurellidge, 1 think he came 

 very near the truth iu suggesting the affinity of this genus 

 with the Fissurellidao. His comparison of the adult Incisura 

 with the post-larval stage of Fissurella is a just one. Almost 

 all the differential external features which serve to distino-uish 

 the adults disappear on comparison of the adult of the one 

 type with the post-larval stage of the other. In the young- 

 Fissurella we see a coiled shell Avitli spiral sculpture, a labral 

 incision of considerable length to the right of the middle line. 



