ORTMANN: THE CRAWFISHES OF THE STATE OF PENNSYLVANIA 431 
An alternative supposition might be entertained. C. limosus might have arrived 
in its present home coming from the south by way of the Atlantic Coastal Plain. 
This, however, does not seem probable. First of all, the distribution of C. limosus 
does not extend southward beyond Virginia, and even in Virginia it is known only 
from a few localities. Southward no representative of this group is known on the 
coastal plain, and, if C. imosus had come from the south, traces of this migration 
might be expected. On the other hand, if it came from the north, as we here 
assume, the fact that it did not spread beyond Virginia may be accounted for by the 
presence of another group of this genus, the subgenus Cambarus (blandingi-section), 
in the southern parts of the coastal plain, which, like C. /imosus, prefers ponds and 
sluggish streams. Indeed both species (C. limosus and C. blanding?) are found actually 
associated at the same localities (by Faxon, 1885a, p. 88, at Trenton, New Jersey, 
and by the writer in the Delaware and Raritan Canal at Princeton, New Jersey), 
but we must bear in mind that in New Jersey, and also in Maryland and Virginia, 
C. blandingi is an intruder, its chief domain being in the Carolinas. 
The following are conclusions from the above considerations : 
Cambarus limosus is an ancient species, characterized by morphological and 
geographical isolation. ‘The most closely related forms are found in Kentucky and 
southern Indiana. An attempt to explain the presence of C. limosus at its present 
location has to connect its range with that of these related species. A connection 
by way of the Atlantic Coastal Plain southward is out of the question. ‘Thus only 
the connection across the Appalachian system remains. The fact that the rivers 
just west of the mountains in western Pennsylvania are occupied by a more 
advanced group of species (propinquus-group) of a subgenus which is certainly of 
Preglacial age, as we shall see below, leads us to the conclusion that the /imosus- 
group also must be not only Preglacial, but older than the propinquwus-group. But 
at that time there was no direct way from the lower Ohio, where its center of dis- 
tribution was situated, into western Pennsylvania and across the mountains, the 
Ohio having no existence as yet, and the general drainage in this region being to 
the north. This leads us to assume a former more northern range of the limosus- 
group, extending into Preglacial Canada; and this assumption furnishes an expla- 
nation why it was possible for the Glacial Epoch to cut the range of the limosus- 
group in two, leaving no representative of it in the region now drained by the 
middle and upper Ohio. Cambarus limosus is a Tertiary type, and it reached its pres- 
ent area coming from the west and by way of the north, being driven south along the 
Atlantic Coastal Plain by the advancing ice of the Glacial Period. It survived during 
the Glacial Period in the region of the lower Delaware River and Chesapeake Bay, while 
