450 MEMOIRS OF THE CARNEGIE MUSEUM 
it has crossed the continental divide, and has invaded the Alleghany River drainage 
in McKean, Warren, and Crawford Counties, and possibly has come down the 
Alleghany River as far as Allegheny County, spreading into some of its smaller 
tributaries.” This assumption seems plausible if we take into consideration only the 
Pennsylvanian material and that from the St. Lawrence Basin. We would have 
here a case of distribution which is entirely unique. C. bartoni robustus should 
then be regarded as a Postglacial form, which originated in the St. Lawrence 
drainage, and in Pennsylvania spread southward, coming from the north. 
But there are objections to this view. C. bartoni robustus has been reported also 
from Virginia, Maryland, and Kentucky, and this, of course, would not be in favor 
of this theory. However, as has been said above (p. 392), I am inclined to believe 
that this southern form is not the same as the northern. If this view should be 
correct, I should regard C. robustus as a good species, and then the above opinion 
would hold good. 
But further, the morphological characters of C. bartoni robustus, as compared 
with those of the typical bartoni, are distinctly more primitive. ‘The shape of the 
rostrum is decidedly more archaic, the original form of the rostrum in the subgenus 
Bartonius being rather elongate, and not short and broad as in C. bartoni. The 
frequent presence of distinct lateral spines on the carapace is undoubtedly a primi- 
tive character; and the ecological peculiarity of preferring larger streams than are 
haunted by the typical form might also be regarded as a remnant of more primitive 
conditions. his, of course, would be strange in a Postglacial form, originating within 
the glaciated area, and we rather ought to expect a higher differentiation than the 
original, typical form. 
Until the question of the identity of our northern ©. bartoni robustus with the 
southern form, which bears the same name, is settled, we cannot form a final opinion. 
If both forms should be actually identical, we might have to deal with two races of 
C. barton, an older one (C. bartoni robustus), which possibly constituted a first wave 
of migration from southwest to northeast, which was overrun and crowded out by 
a later wave, consisting of C. bartoni typicus. Remnants of the older stock have 
been able to survive only at a few, scattered localities in the south, while in the 
5 The Alleghany River, between Sandy Creek and Verona, has been investigated repeatedly. It is a curious fact 
that Dr. D. A. Atkinson collected here a large number of C. obscurus on September 17, 1900, but not a single robustus. 
I was at the same place on June 1, 1904, together with Dr. Atkinson and Dr. O. T. Cruikshank, but we did not collect 
this form (conditions were unfavorable) ; on November 19, 1904, I spent a whole day there, collecting numerous C. ob- 
scurus, and a few C. bartoni (typical), but not asingle robustuswas seen. When I visited this place again, on September 
7, 1905, I secured within a short time six specimens of C. bartoni robustus, and on September 30, 1905, I found three fine 
specimens a little further up the river, at Hulton, although I did not hunt very diligently. Is it possible that the 
migration of this form down the river is going on? Doesit gradually become more abundant ? 
