the path of the excurrent stream of the ])raiichial cavit}' m 

 these forms by the deepening of the slit or its conversion 

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



ScissarcUa is cei'tainly one of the most primitive Proso- 

 branchs known and may be more nearly related to the 

 Docogiossa than are the other groups named, but even in 

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

 in PIctirotonKirki, etc. 



The fossil Bellerophontidn' were very primitive Gastropods, 

 though we cannot know enough about them to say much of 

 their relationships. It is, however, po;-sible to conceive of 

 the evolution of the Docogiossa from forms something like 

 them, the symmetrical spire undergoing relative reduction as 

 the posterior margin of the peristome grew out. 



