BULL. GEOL. SOC. AM. 



VOL. 24, 1912, PL. 12 



PLATYSTROPHIA PONDEROSA VAR. STEVENSONI GRABAU 



1. Cardinal view of a cast (plastotypei from a rock mold. 



2. Internal rock mold, showing the usual form of occurrence. 



3. View of another specimen from the brachial valve side, showing natural median 

 fold and plications. 



All the specimens are natural size. Types in collection of Columbia University. 



