Jan., 1912.] 
The Arnheim Formation. 
449 
coarse grained, more or less cross bedded limestones at the base 
of the Arnheim section, in the southern part of Jefferson county, 
in Kentucky, and thence southward to the northern part of Nelson 
county, suggest the presence of strong, irregular currents, but 
do not necessarily indicate an elevation of the sea bottom. 
These coarse grained limestones may represent in part the lower 
division of the Arnheim, east of the Cincinnati geanticline. 
The sudden influx of Leptaena richmondensis , Rhynchotrema 
dentata , and Dinorthis carleyi during the deposition of the lower 
part of the upper or Oregonia division of the Arnheim, suggests 
the lowering of some barrier which for long geologic periods had 
kept any representatives of this group of species from Cincinnatian 
areas. The most astonishing feature of this faunal immigration 
is its great geographical range compared with its extremely short 
duration. What were the favorable conditions which in a brief 
time permitted this fauna to reach points as remote as southern 
Tennessee and southwestern Ohio? What were the unfavorable 
conditions which with equal suddenness caused the disappearance 
of this fauna? Where was the basin from which this fauna 
entered the Cincinnatian areas? 
14. Origin of Arnheim faunas. 
These questions are easier asked than answered. For instance, 
the general geographical distribution of Dinorthis carleyi in south- 
western Ohio, southeastern Indiana, and western Kentucky, as 
far south as Nelson county, suggest its origin from some northern 
source, until it is remembered that this species occurs also at 
Clifton, in southwestern Tennessee. The northern origin of 
Dinorthis carleyi is favored also by the greater abundance of this 
species and by its greater vertical range in southwestern Ohio com- 
pared with its occurrence in southeastern Indiana, western Ken- 
tucky, or southwestern Tennessee. Moreover, the species attains a 
larger size and the valves are thicker as a rule in Ohio. In most 
of Kentucky, and at almost all localities in western Tennessee, 
from which the Arnheim is known, Dinorthis carleyi is absent. 
These facts suggest that the conditions were much more favorable 
for the growth of this species northwards, rather than southwards. 
As a matter of fact, however, the Arnheim is absent also along 
Lake Huron and Lake Ontario, and no trace of it has been recorded 
from Wisconsin or Minnesota on the northwest, nor from Pennsyl- 
vania or New York on the northeast. Dinorthis carleyi is so 
closely related to Dinorthis retrorsa from the Bala group of Wales 
that it certainly must be regarded as a derivative, but by what 
path did it enter Cincinnatian areas ? Billings figured a specimen 
of Dinorthis retrorsa from the Trenton in the vicinity of Ottawa, 
in Canada, but this species is not mentioned by Dr. Ami, in any of 
his more recent studies from this locality. 
