454 MEMOIRS OF THE CARNEGIE MUSEUM 
found at Morgantown, Monongalia County. It undoubtedly goes further south in 
West Virginia, but no records are at hand from these parts. 
The writer was unable to discover this species in the state of Ohio (Harrison, 
Carroll, and Stark Counties), and its absence north of the Ohio-Alleghany River is 
well established (with one exception to be presently mentioned). Particular pains 
have been taken to ascertain the latter fact. While it is very abundant in Alle- 
gheny County, south of the Alleghany and Ohio Rivers, the writer has not in a 
single instance found it north of them. He has searched in vain at many localities 
in northern Beaver, northern Allegheny, in Armstrong, and Butler Counties, and 
further north. At one single locality, however, on the northern side of the Alle- 
ghany River it is present. It was found by Dr. D. A. Atkinson near Squaw Run, 
at Aspinwall, Allegheny County (more correctly near Claremont). This seems 
to be a very restricted locality. The writer did not visit it himself, but he hunted 
all over the region around it from Aspinwall to Squaw Run, and beyond to Mon- 
trose, Powers Run, and Harmarville, without discovering additional localities for 
the species. Thus it seems that this locality is the only one on the northern side of 
the river, and we are able, as we shall see below, to explain its presence there. 
This species is generally found at elevations from 900 to 1,200 feet ; and it rarely 
descends to 800 feet or less. The lowest altitude at which it was found is 790 to 800 
feet at Colliers, Brooke County, West Virginia, and at about the same (estimated) 
elevation it occurs in Fern-Hollow and Nine-Mile Run, Pittsburgh. 
b. Origin of the distribution of C. monongalensis. 
The distribution of this species outside of the state is very incompletely known, 
and consequently we cannot form any opinion as to its center of dispersal. Con- 
sidering, however, that it is clearly a form cognate to C. carolinus, we may safely 
assume that it also came from the south, from West Virginia. C. carolinus and C. 
monongalensis seem to be two parallel species, closely connected genetically, the 
one belonging to the Old Tertiary base-level within the mountains, (elevation 1,200 
to 2,000 feet), the other to the hills west of Chestnut Ridge (elevation 900 to 1,200 
feet), formed by the Tertiary erosion of this base-level. The areas of both are sepa- 
rated by the escarpment of the Chestnut Ridge, and both have probably migrated 
on parallel lines. 
C. monongalensis must have invaded Pennsylvania and the Panhandle of West 
Virginia from the south, being confined to the region between the Chestnut Ridge 
and the Ohio River. ‘That in this case a large river forms a barrier to an aquatic 
creature is highly interesting, but is easily explained by the ecological habits of the 
