34 . GILBERT C. BOURNE. 



it is not exact to say, as Hedley has, that Incisura represents 

 an arrested stage of developtnent of a Fissurellid. It is a 

 more reasonable inference from the facts that the two 

 families have descended from a common stock, and have 

 diverged in different directions. There are several arguments 

 in favour of this inference. One which in my opinion has 

 great weight is derived from the condition of the left kidney 

 in the two families. In the Scissurellidae, as I have shown, 

 the left kidney is relatively of large size, and is a true 

 " papillary sac,^' phagocytic in function like the left kidney 

 of the Pleurotomariidee, Haliotida), and Trochidae. In the 

 Fissurellidas this organ is reduced to a mere rudiment, and 

 may, I believe, disappear altogether in some species, for I 

 have failed to find a trace of it in transverse and horizontal 

 sections of F. gra3ca. 



Remy Perrier (14) has stated that the epithelium of the 

 left kidney of Fissurella is identical with that of the right 

 kidney, but there is some doubt about this, and a renewed 

 investigation of the left kidney of several species of the 

 Fissurellidae is much to be desired. But there is no doubt 

 that it is a vestigial organ, and that in this respect the 

 Fissurellidae have been specialised along a different line to 

 the Scissurellidae, which have retained the left kidney in a 

 fully functional state. Per contra, while the Fissurellidae 

 retain to a large extent the primitive scalariform character of 

 the pedal centres, the Scissurellidae have in this respect sur- 

 passed them in specialisation, for their pedal centres are 

 concentrated to a degree elsewhere unknown among the 

 Rhipidoglossa. The divergence of the two types is obvious, 

 and one may conclude that both have been derived from a 

 stock very nearly represented by the so-called Emarginuliform 

 larva of Fissurella, which had a spirally coiled shell with a 

 large umbilicus, spiral sculpture and a considerable labral 

 incision. A corneous multi-spiral operculum and a well- 

 developed epipodial ridge bearing sub-ocular as well as 

 posterior epipodial tentacles were present. The left kidney 

 was a well-developed papillary sac, and the pedal centres were 



