430 MEMOIRS OF THE CARNEGIE MUSEUM 
sylvania or New York, the advancing ice of the Glacial Period must have entirely 
covered a large part of this range. In the central parts, in Ohio and western Penn- 
sylyania, it was impossible for these forms to retreat southward, these parts being 
occupied by another vigorous group of river-crawfishes, as we shall see below (pro- 
pinquus-group), and only in the east and west a chance to survive was left. The 
eastern remnant is the present C. /imosus, the western is the group of species found 
now in southern Indiana and Kentucky. 
How C. limosus reached the Atlantic Coastal Plain from the Erigan basin is very 
hypothetical. One suggestion may be made. Not only does the North Branch of 
the Susquehanna seem to be a reversed river, but the West Branch has captured 
a large part of the original drainage of the Alleghany Plateau in Potter, Cameron, 
and Clearfield Counties. Davis (1889, p. 248) believes that this happened largely 
in Pretertiary times, since he thinks that the Alleghany Plateau belongs to the 
Cretaceous peneplain. However, Campbell (1903, p. 280) has shown that there 
are two old base levels in northern Pennsylvania, an older one (Cretaceous), iden- 
tical with that of Davis, and a younger one (1,600 to 2,200 feet) corresponding to 
the Harrisburg peneplain of Old Tertiary age. Since the headwaters of the West 
Branch of the Susquehanna are carved into this second peneplain, it is probable 
that during Tertiary times the stream-piracy of the Susquehanna was going on 
rather vigorously. If we assume that C. limosus in Tertiary time existed in this 
part of the Erigan River drainage, namely in the Old Upper and Middle Alleghany 
Rivers, which did not belong to the Old Monongahela or Spencer River, it must 
have been possible for it to get into the Susquehanna drainage in consequence of 
this stream-piracy in Tertiary times. This, however, is a mere suggestion. There 
is no other evidence for it but the bare fact that stream-piracy has gone on in this 
region. J mention it here only to show that the crossing over of this species into 
the Atlantic drainage is not altogether unthinkable. 
After arriving in the coastal plain C. limosus was cut off in the Glacial Period 
from its allied forms in the west. But it survived, and in Postglacial times was 
able to advance again. But the Postglacial dispersal cannot have amounted to 
much, since the increasing roughness of the streams, caused by the Postglacial eleva- 
tion of the country, was not favorable to a northward expansion. We do not know 
the exact northern boundary of OC. limosus outside of our state. It is found in New 
Jersey as far north as Morris County, yet we do not know whether it reaches Rari- 
tan and New York Bays, and the Hudson River. No positive record from New 
York State is at hand (see De Kay, 1844, p. 23, and Paulmier, 1905, p. 117). 
“See: Carll, 1880, pp. 333 and 336, map, Pl. 2; Leverett, 1892, pp. 129 and 132 ; Tight, 1903, map, Pl. 1. 
