•21)0 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



the path of the excurrent stream of the branchial cavity in 

 these forms by the deepening of the slit or its conversion 

 into a hole at or near the apex of the conical shell. 



Scissurella is certainly one of the most primitive Proso- 

 hranchs known and may he more nearly related to the 

 J)ocoglossa than are the other groups named, but even in 

 this form the left kidney is specialised in the same way as 

 in Pleurotomaria, etc. 



The fossil Bellerophontidas were very primitive Gastropods, 

 though we cannot know enough about them to say much of 

 their relationships. It is, however, possible to conceive of 

 the evolution of the Docoglossa from forms something like 

 them, the symmetrical spire undergoing relative reduction as 

 the posterior margin of the peristome grew out. 



