Jan., 1912.] 
The Arnheim Formation. 
45 1 
its precursors already in the Trenton. The Trenton of New York, 
and New Jersey, for instance, contains a species of Rhynchotrema 
which is sufficiently 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 ponder osa 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 very abundant during the Corryville, and locally also during 
the Mount Auburn. In the Mount Auburn it is found in great 
numbers from 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 
Leptaetia richmondensis , Rhynchotrema dentata, and Dinorthis 
carleyi , 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 Montgomery 
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 Lep- 
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- 
