43° 



The Ohio Naturalist. 



[Vol. XII, No. 3, 



to give special designations to these divisions; the upper Arnhcim 

 being called the Oregonia division, and the lower Amheim, the 

 Sunset division. 



The ease with which the two divisions of the Arnheini may be 

 traced is due largely to the fact that the Oregonia division includes 

 a characteristic fauna which may be identified readily even by a 

 beginner in the study of fossils. This fauna includes Dinorthis 

 carleyi, Rhynchotrema dentata var., Leptaena richmondensis va 

 and Platystrophia ponder osa. BH 



In Ohio and Indiana Dinorthis carleyi recurs in a slightly 

 different form at the base of the Blanchester division of the 

 Waynesville member; Rhynchotrema dentata is found in the 

 Blanchester division of the Waynesville member and near the 

 upper part of the Whitewater member; and Leptaena richmonden- 

 sis ranges from the Clarksville division of the Waynesville to the 

 top of the Whitewater. But none of these fossils is found asso- 

 ciated with Platystrophia ponder osa anywhere except in the lower 

 part of the Oregonia division of the Arnheim. It is this association 

 of fossils which gives them such great value as horizon markers. 

 Moreover, if attention be confined to Kentucky, then Dinorthis 

 carleyi and Rhynchotrema dentata here occur only in the Oregonia 

 division, and within this limited territory they are strictly charac- 

 teristic of the upper Arnheim. 



