iNcisuRA (scissurella) LYTTELTONENSIS. ,'33 



phageo-intestinal groove in the capacious stomach, the 

 existence of numerous hepatic ducts, are all points in which 

 lucisura agrees with Fissurella, and differs, to a greater or 

 less degree, from the Pleurotomariidee, Haliotidae, Trochidge, 

 and Turbonid^. The presence of a right and left columellar 

 muscle in the Scissurellidee is evidently an antecedent stage 

 of the horse-shoe shaped columellar muscle of the Fissurellidae. 



The eyes, which are open in Pleurotomariidee, Haliotid89, 

 and Trochidge, are closed in both the Scissurellidae and the 

 Fissurellidge. 



The subocular and posterior epipodial tentacles of the 

 Scissurellidse are paralleled by the similar larval organs in 

 the Fissurellidse. 



In both the Scissurellid^ and Fissurellid^ the increased 

 size of the last whorl of the shell and the diminution of the 

 visceral spire has led to a broadening of the dorsal part of 

 the body, in consequence of which the bases of the ctenidia 

 are widely separated on the right and left sides of the body, 

 the pericardium is transversely elongated, and the heart and 

 kidneys are shifted towards the mid-dorsal line in the roof 

 of the mantle-cavity. In these respects Incisura is inter- 

 mediate between Fissurella and the other families of Rhipido- 

 glossa enumerated above. 



The nervous system of Incisura, though much specialised, 

 shows more resemblance to that of the Fissurellidas than to 

 that of any other Rhipidoglossa, as has been explained in 

 detail in the descriptive part of this paper. The corre- 

 spondence in the labial commissure, the buccal ganglia, and 

 the visceral commissure is very exact. The pedal centres of 

 the Scissurellidee have undergone great concentration, but 

 this is foreshadowed in the pedal cords of the Fissurellidae, 

 which are much shortened in comparison with the elongated 

 scalariform pedal centres of such families as the Pleuroto- 

 mariidge, Haliotidge, and Trochidge. 



There can be little doubt, then, as to the affinity of the 

 Scissurellidas with the Fissurellidae, but the exact relationship 

 of the two families remains to be considered. In my opinion 



VOL. 55, PART 1. NEW SERIES. 3 



