8 



( turnings — Morphogenesis of Platystroph 



%a. 



of the species which are here recognized as belonging to 

 Platystrophia are marked by important differences at this 



stage. 



3 



eo xi 



l\!P<3*e> xii 



XVI 



Fig. 3. Series of specimens of Platystrophia lynx from Vevay, Indiana, 

 showing change in contour, profile, and convexity of shell with advancing 

 growth. I, specimen l mm broad, nepionic stage, natural size, same speci- 

 men as fig. la. V, beginning of neanic stage, in which the dorsal fold 

 and ventral sinus become defined. The early ephebic stages are represented 

 by numbers XI to XVI, the latter being a nearly adult individual, possess- 

 ing four plications in the sinus. The method of origin of the plications 

 of the sinus is shown to the left of each row. All natural size. Author's 

 collection.* 



* In all the outline figures in this paper the lines correspond to the depres- 

 sions between plications, and the interspaces to the plications themselves. 



