Cinc'uinatian and Lexington Fossils 57 



sfrophia rcversata, Foerste. from the Brassfield or Clinton bed of 

 Ohio is another mutation in which the fission of the pHcations is 

 confined to the fold and sinus. 



Platystrophia colbiensis — precursor, var. nov 



(Plate IV, fig. i.) 



By selection from a large number of PlatysfropJiia colbiensis, 

 it is possible to difTerentiate a series of specimens characterized by 

 a larger size, greater width, less convexity, in which the posterior 

 part of the shell along the hinge-line frequently is slightly extended 

 beyond the width of the shell across the middle. The width of an 

 average sized specimen is 27 mm. ; the length, 19 mm. ; the con- 

 vexity, 14 mm. Four plications occupy the fold, and 8 to 10 are 

 found on the sides. 



Specimens having these characteristics occur between Colby 

 and Winchester, and also at various localities between Millersburg 

 and Pleasant Vallev, Kentucky. Specimens in which the hinge-line 

 is slightly shorter than the width across the middle of the shell also 

 occur. 



Specimens of Platystrophia pondcrosa make their appearance 

 as early as the upper part of the Fairmount bed. 'associated with 

 Orthorhynchula linneyi, along the eastern side of the Cincinnati 

 geanticline, from Lincoln to Clark county. They may be traced 

 at this horizon as far north as Maysville. Kentucky. Both forms 

 with the hinge-line slightly longer than the width across the 

 middle of the shell, and those with shorter hinge-lines occur one 

 mile north of Paint Lick. These specimens differ from typical 

 Platystrophia ponderosa chiefly in their smaller size. Specimens 

 an inch and a cjuarter wide, resembling Platystrophia ponderosa, 

 occur also just beneath the massive Garrard sandy limestones at 

 the George Million locality, southeast of Million, Kentucky, and 

 similar specimens are found near the base of the Upper Eden at 

 Maysville. These low^r specimens agree with Platystrophia pon- 

 derosa in possessing a low fold not laterally compressed, the 

 number of pHcations being four on the fold, increasing occasionally 

 to 5 or 6. If these forms actually belong to the Platystrophia pon- 

 derosa series, as they appear to do, the latter species may be of 

 indigenous origin, and may have been derived from the larger 

 varieties of the Platystrophia colbiensis type of shell. 



