Jan., 1912.] The Arnheim Formation. 451 



its precursors already in the Trenton. The Trenton of New York, 

 and New Jersey, for instance, contains a species of Rhynchotrema 

 which is sufficient!}" like the Arnheim form to have served at least 

 as an ancestral form. But this does not furnish a hint as to the 

 direction from which the Arnheim fauna invaded the Cincin- 

 natian areas. 



Platystrophia ponderosa is another species which appears to 

 have entered the Cincinnatian areas from the south. The species 

 may have been of indigenous origin. It certainly is known in 

 Kentucky as early as the Fairmount, if not the upper Eden, and 

 was ver}' abundant during the Corryville, and locally also during 

 the Mount Auburn. In the Mount Auburn it is found in great 

 numbers froin the eastern half of Hamilton and Butler counties, 

 in Ohio, southeastward to Maysville, in northern Kentucky, and 

 thence southward toward Lincoln county. It is very rare at the 

 Mount Auburn horizon in most parts of Indiana and in the adja- 

 cent parts of western Kentucky. During the lower or Sunset 

 division of the Arnheim it occurs apparently only in the dark 

 blue, argillaceous limestone between the southern part of Fleming 

 county and the eastern part of Montgomery county. It occurs 

 here in such small numbers that it is difficult to believe that it 

 could have spread during the upper Arnheim over by far the larger 

 part of the Cincinnatian areas from such a restricted source. 

 During the earlier part of the upper Arnheim, before the advent of 

 Lcptaena richmondensis, Rhynchotrema dentata, and Dinorthis 

 carlcyi, it spread over southwestern Ohio, and along the eastern 

 side of the Cincinnati geanticline as far south as Maysville, 

 Kentucky. 



Along the more southern exposures on the eastern side of the 

 Cincinnati geanticline, from the eastern part of Montgoinery 

 county, in Kentucky, southward to Lincoln, and westward to 

 Marion county, Platystrophia ponderosa occurs distinctly above the 

 Leptaena-Rhynchotrema horizon. Between the more northern 

 areas, in which Platystrophia ponderosa occurs beneath the Lcp- 

 taena richmondensis horizon, and the more southern areas in 

 which Platystrophia ponderosa occurs chiefly above the Leptaena 

 horizon, there is an intermediate area, on both sides of the gean- 

 ticline, in which Platystrophia ponderosa and Leptaena richmon- 

 densis occur together, in the same restricted zone, near the base 

 of the upper or Oregonia division of the Arnheim. The anomalous 

 association of these fossils at Collinsville, in the northern part of 

 Butler county, has been mentioned already. 



The occurrence of Stromatocerium, in the Arnheim is limited 

 to a relatively small area in central Kentucky, where, usually, it 

 is quite rare excepting at a few localities. A single locality is 

 known, in the southern part of Fleming county, where Stroma- 



