42 ( hi m ings — Morphogenesis of Platystrophia. 



21 



O"" 



Fig. 21. a, &, c, d, /, Platystrophia biforata from the Clinton of 

 Rochester, N. Y. ; a, b, c, dorsal and d. f, ventral valves : adjacent diagrams 

 show method of origin of plications of fold and sinus, e, P. biforata 

 from Wesenberg, Russia (Ordovician) ; e, ventral ; e', dorsal ; e", posterior ; 

 e" , anterior views, a, 6, c, d, /, collection of Mr. C. J. Sarle, Rochester, 

 N. Y. ; e, Yale Collection. 



Fig. 22. Series of Clinton Platystrophia biforata showing variation in out- 

 line and index. Value of index given to right of each shell. Collection of 

 Mr. C. J. Sarle, Rochester, N. Y. 



Fig. 23. Specimen of Platystrophia biforata from the Clinton of Gasport, 

 N. Y., showing growth varices 7-7 and the stages separately drawn (b-f). 

 Collection U. S. National Museum. 



