452 MEMOIRS OF THE CARNEGIE MUSEUM 
b. Origin of the distribution of C. carolinus. 
Generally, conforming to the subgenus Bartonius, the center of radiation of this 
species is to be sought in the southern part of the Appalachian system. It has fol- 
lowed in its migration the strike of the mountains, keeping to the higher parts of 
the latter. Thus it has entered southern Pennsylvania, being restricted here to the 
highest portions of the Allegheny Plateau. 
The lowest elevation at which I found it is at Ohiopyle, Fayette County, 1,250 
feet, and at Dunbar, Fayette County, 1,260 feet. (At the latter place a few strag- 
glers — two specimens — were taken as low as 1,070 feet, associated with C. diogenes, 
but here they had apparently come down from the top of the mountain, where this 
species was abundant at 1,260 feet.) All other localities in Pennsylvania were 
higher, generally between 1,500 and 2,000 feet. 
The northern boundary of this species in our state is formed by two different, 
opposite features in the physical geography. Between Chestnut and Laurel Hill 
Ridges it is a cross divide of the longitudinal valley ; between Laurel Hill and the 
Alleghany Front the deep erosion of the original longitudinal valley by the head- 
waters of the Conemaugh River forms the boundary. We do not know much of the 
geological history of this region, but it seems to me that the floors of these high 
valleys with their extensive clay deposits form a part of a former base-level, namely, 
that of the Old Tertiary peneplain identified with the Harrisburg peneplain by 
Campbell (1903, p. 293). In northern Somerset and southern Cambria Counties 
this has been eroded by the Conemaugh system, thus removing a good deal of the 
clay bottoms, which seem to be an essential condition for this species, and conse- 
quently the lack of this feature, or its interruption by the Conemaugh system at the 
northern end of Somerset County, has formed here the barrier to the dispersal of C. 
carolinus. — 
To all appearances C. carolinus is a Postglacial immigrant into this state. The 
northern boundaries in both of the longitudinal valleys are rather insignificant, and 
we should expect that C. carolinus, being a chimney-builder and able to leave the 
water for a considerable time, should be able, like" C. bartoni, to cross boundaries of 
this character. We should even expect that it would be better fitted to do so than 
C. bartom. In fact C. carolinus must have done so repeatedly on its way from the 
South, being found in the upper drainages of rivers running in different directions, 
for instance, the upper Youghiogheny in Maryland, the upper Potomac in Mary- 
land and West Virginia, upper Decker’s Creek (tributary of the Monongahela), and 
upper Cheat River in West Virginia.’ That it has been checked in Pennsylvania 
®! As to stream adjustments and migration of divides in Garrett County, Maryland, See Abbe, 1902, p. 47, 53. 
