446 MEMOIRS OF THE CARNEGIE MUSEUM 
C. propinquus and of C. propinquus sanborni is rather unsatisfactory, but none of 
the known facts is opposed to our theory. With reference to C. obscwrus in Penn- 
sylvania, I think our assumptions are well supported. Cambarus obscurus is a Pre- 
glacial form, belonging to the Old Monongahela (or Spencer) River, which survived dur- 
ing Glacial times in the headwaters of this river (Lake Monongahela), and spread out, 
in Postglacial times, over the whole of the Upper Ohio drainage (in addition to the 
Ohio and Monongahela, over the drainages of the Beaver and the Alleghany Rivers), and 
was only checked in its dispersal in the direction toward the mountains by the roughness 
of the streams. The Ohio River of Postglacial times opened a way down stream, 
but C. obscwrus was unable to spread in this direction, since these parts were occu- 
pied by another closely allied species, C. propinquus sanborni. It has slightly 
entered upon the territory of the latter (Fishing Creek), but has not been able to 
crowd it out or to conquer it. Similar conditions prevail in the Lake Erie drainage, 
which has been reached in consequence of stream-piracy, or else, by the help of 
modern canals. Here it came into contact with C. propimquus. In both cases (in 
West Virginia and Erie County, Pennsylvania) we see that the other species show 
indications of an inclination toward C. obscwrus. I believe we have to deal here with 
hybridization, but this will be discussed later. Finally the species has crossed over 
into the Lake Ontario drainage in the region of the headwaters of Genessee River, 
presumably in consequence of stream-piracy. In the upper part of this system, in 
Pennsylvania, it did not find any competition, and is alone represented there, while 
in the lower part, at Rochester, N. Y., it is again found associated with C. propin- 
quus. Further details with respect to these parts are not at hand. 
Comparing the distribution of the propinquus-group with Adams’ scheme of 
Postglacial dispersal of the biota of North America, we see at a glance that the whole 
group belongs to his northeastern biota of the second wave (Adams, 1905, p. 58). The 
biotic preserve of this element, during glacial times, was not far from the southern 
edge of the ice, in what is now the Ohio drainage, but it was restricted to this 
western part, and was not extended east of the Alleghany Mountains. In Post- 
glacial times this group advanced northward, forming part of the second wave, 
which is most clearly seen in the present distribution of C. propinquus, which largely 
entered the coniferous forest-belt in Michigan, New York, and Canada. ‘The other 
two forms (C. propinquus sanborni and C. obscwrus) did not take much part in the 
migration of this wave, since they found a barrier to the north in the shape of the 
continental divide, and then, after they had crossed this divide at certain points, 
they found competition in C. propinquus, which had populated the whole St. 
Lawrence drainage at an earlier date (‘‘biocenotic barrier”’). In New York state, 
