454 MEMOIRS OF THE CARNEGIE MUSEUM 



found at Morgaiitown, Monongalia County. It undoubteilly goes; further south in 

 West Virginia, hut no records are at hand from these parts. 



The writer was unable to discover this species in the state of ( )hio (Harrison, 

 Carroll, and Stark Counties), and its absence north of the Oliio-Alleghany Kiver is 

 well established (with one exception to be presently mentioned). Particular jtains 

 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 Ai-mstrong, 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 l)y I>r. U. A. Atkinson near Squaw Run, 

 at Aspinwall, Allegheny County (more correctly near Claremont). This seems 

 to l)e a very re.stricted locality. The writer did not visit it himself l>ut he hunted 

 all over the region aromid 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 <S00 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. 



h. Origin of ihe (Jisfrihiitii))/ of C. moi/onijalcims. 

 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. ( 'on- 

 sidering, however, that it is clearly a form cognate to (J. carol iniis, we may safely 

 assume that it also came from the south, from West Virginia. C. carol iiii(>^ and ('. 

 moiionyaJcnsis .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 l)y the Tertiary erosion of this base-level. The areas of both are sepa- 

 rated by the escarpment of the Chestnut Ridge, and l)oth have probably migrated 

 on parallel lines. 



C. mononijalcniii-'t 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 



