Jan., 1912.] The Arnheirn Formation. 449 



coarse grained, more or less cross bedded limestones at the base 

 of the Amheim 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 ma}^ represent in part the lower 

 division of the Arnheim, east of the Cincinnati geanticline. 



The sudden influx of Leptaena richmondensis, Rhynchotrema 

 dcnlata, 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. 



