1^8 A^ig- P- Focrste 



Platystrophia profundosulcata, Meek. 



{Plate VI, figs. 15 A-C.) 



Platxstrophia profundosulcata is a much smaller shell than 

 Platystrophia laticosta and occurs usually at a lower horizon. It is 

 very abundant at some localities in the lower Fairmount. especially 

 near Cincinnati, Ohio, although numerous specimens occur also 

 in the Mount Hope. In typical specimens, the sinus is much deeper 

 and the fold stronger than in typical specimens of Platystrophia 

 laticosta of the same size. Mature specimens average about i8 

 mm. in width. The leng-th equals between seven and eight-tenths of 

 the width, and the thickness varies between six and eight-tenths 

 of the width. The hinge-line frequently is shorter than the width 

 of the shell across the middle, but may equal the latter or even 

 slightly exceed it. Specimens in which the lateral plications of the 

 fold and sinus are almost or entirely obsolete are rare. Viewed 

 from the side, the shell has a rather gibbous appearance. Usually 

 6 or 7 plications occur on each side of the fold, but specimens widi 

 5 plications are not rare. 



The specimens from the Mount Hope bed (plate IV, fig. 4) 

 are closely similar to those from the lower Fairmount, but usually 

 have the fold less strongly elevated and the sinus less profound. 

 They are wider, and less convex. The width frequently reaches 

 20 mm. The numl^er of plications usually is 6 or 7 but may vary 

 from 5 to 10. At the Mount Hope horizon, this species usually 

 occurs alone, not associated with any other form of Platystrophia. 

 In the Fairmount. it may be associated with Platystrophia laticosta. 



Platystrophia laticosta, Meek. 



(Plate III, figs. I A. B.) 



Platystrophia laticosta is characterized by the presence of 5 

 to 7 broad plications on each side of the fold. Both the fold and 

 sinus are less compressed laterally than in Platystrophia cypha, 

 and the lateral plications on the sides of the fold and sinus, though 

 inconspicuous, are less commonly obsolete. The postero-lateral 

 outline usually does not form angles more acute than 70 degrees. 

 The shell is comparatively compressed antero-posteriorly. The 

 typical specimens occur in the Fairmount bed, at Cincinnati. Ohio, 

 and at corresponding horizons elsewhere, but similar forms occur 

 also in the Bellevue bed at Madison, Indiana. Cincinnati, Ohio, 



