414 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 56. 



PLATYSTROPHIA COLBIENSIS Foerste. 



Plate 44, figs. 8-11. 



Platystrophia colbiensis Foerste, Bull. Sci. Lab. Denison Univ., vol. 16, 1910, 



p. 55, pi. 4, figs. 2A-B. 

 Platystrophia of colbiensis Foerste, Journ. Cincinnati See. Nat. Hist., vol. 21, 



1914, p. 131. 



In the Upper Trenton of Kentucky and Tennessee occurs the species 

 which Doctor Foerste has described as Platystrophia colbiensis. The 

 fold is slightly elevated and slightly compressed; the sinus is shallow. 

 Four subequal plications occupy the fold; they have originated by 

 the bifurcation of the two primary pUcations at the beak. There are 

 three subequal plications in the sinus; one is primary and occupies a 

 median position; two are lateral, secondary plications which are im- 

 planted on the slopes of the sinus at about 1.5 mm. from the beak. 

 There are 9 to 11 plications on the lateral slopes. Young specimens 

 have subequally convex valves, but the brachial valve of older speci- 

 mens is quite ventricose. 



This species is larger than Platystrophia amoena new species and 

 the fold is slightly compressed, while the fold of that species becomes 

 progressively wider as it approaches the frontal margin. 



Measurements. — 20.6 mm. along the hinge line, 21.6 mm. wide 

 across the middle, 15.3 mm. high, 14.4 mm. thick, 9 mm. sinal width, 

 6.5 mm. sinal depth, 2 mm. fold depth. 



Occurrence. — Trenton group: Cynthiana limestone : Between Colby 

 and Winchestei , Paris, Lexington, Frankfort, etc., Kentucky. Catheys 

 formation: Edgefield Junction and Nashville, Tennessee. 



Plesiotypes.~Cat. Nos. 34231, 48614, U.S.N.M. 



PLATYSTROPHIA COLBIENSIS-MUTATA Foerste. 



Plate 44, fig. 12. 



Platystrophia colbiensis mutata Foerste, Bull. Sci. Lab. Denison Univ., vol. 16, 

 1910, p. 56, pi. 4, figs. 3a-6. 



Among the specimens representing Platystrophia colbiensis there are 

 thinner and somewhat higher and broader shells with more than four 

 plications on the fold and more than three in the sinus. The number 

 varies generally from five to six on the fold and four to five in the 

 sinus. The increase is effected b}^ the bifurcation of the lateral pli- 

 cations of the fold and by implantation on the slopes of the sinus. 

 One specimen has eight plications on the fold and seven plications in 

 the sinus. In this case both the median and lateral plications of the 

 fold have bifurcated and tertiary plications have been implanted on 

 the slopes of the sinus and between the primary and secondary 

 plications. 



