64 -4^'^- P- Foerste 



cord, Kentucky. Thc}^ range as high as the upper or Blanchester 

 division of the Waynesville bed at Moores Hill, Indiana (plate iv, 

 fig. 13 A.B), and at Woodville, in Clermont county, Ohio. 



Possibly Platystrophia acutilirata was derived from the Platys- 

 frophia cypha stock. In that case the process of evolution demands 

 a reduction in the height of the fold and an increase of the size 

 and elevation of the lateral plications of the fold ; in other words, a 

 reversal in part to former conditions. The tendency toward pro- 

 longation along the hinge-line and toward a larger number of 

 lateral plications on each side of the fold than in Platystrophia 

 laticosta remains. The possibility of the derivation of Platystro- 

 phia acutilirata in a more direct line from Platystrophia laticosta 

 must be considered elsewhere. 



The typical characteristics of Platystrophia cypha are the high 

 median fold of the brachial valve, the deep sinus of the pedicel 

 valve, with a tendency toward the disappearance of the two exterior 

 plications from the group of four plications occupying the fold, 

 and from the group of three plications occupying the sinus. AAHiile 

 the shell is prolonged into a spine-like projection along the hinge 

 in the type specimen, this is an extreme form of the species. In 

 the Arnheim bed, half a mile south of Smithville, in Bullitt county, 

 Kentucky, and at corresponding horizons elsewhere in Kentucky, 

 west of the Cincinnati geanticline, a form of Platystrophia occurs 

 (plate iv, figs. 7 A.B, and 14 A,B.) which may represent a line of 

 development from Platystrophia cypha toward Platystrophia acuti- 

 lirata. In these specimens the sides of the shell, on each side of 

 the fold and sinus, on ventral view, appear compressed instead of 

 inflated as in typical Platystrophia cypha. However, some of the 

 specimens show a fairly conspicuous fold and sinus, and there is 

 a tendency toward the disappearance of the exterior pair of plica- 

 tions of the fold and sinus (figs. 7 A,B), while in other specimens 

 the elevation of the fold and the depth of the sinus are distinctly 

 less, the exterior plications of the fold and sinus are more con- 

 spicuous, (figs, 14 A.B) and this type of shell may have led to 

 Platystrophia acutilirata. There is a considerable variation in the 

 number of lateral plications, but these tend to be numerous in the 

 broader shells. For shells of this type the name Platystrophia 

 cypha-conradi is proposed. 



Platystrophia cypha-conradi appears to have been a precursor 

 of the type of shell which is very prolonged along the hinge-line^ 



