ORTMANN: THE CRAWFISHES OF THE STATE OF PENNSYLVANIA 461 
it should have been able to cross the latter. Consequently its range extends 
beyond that of C. monongalensis. 
It is doubtful what physical feature constitutes the northern boundary of this 
species. In Jefferson, Armstrong, and northern Butler Counties, where the boun- 
dary is apparently located, the late Tertiary base-level, to which this species 
belongs, loses its identity. Possibly it was not developed at all, and this region was 
not reduced to base level. So it might be possible that the roughness of the coun- 
try constitutes a barrier here, and this is supported by the fact that the boundary is 
located further south in the Alleghany valley than to the east and west of it. On 
the plateau-like regions in Indiana and Butler Counties, favorable localities are 
abundant, while the narrow Alleghany valley, with the deeply cut valleys tributary 
to it, do not offer congenial conditions. 
It is different further west. In Lawrence and Mercer Counties this species has 
invaded the glaciated area, and is found to the north of the terminal moraine 
(see Lewis, 1884, p. 183 and 193, Pl. 11 and 12), and here prefers the swampy depres- 
sions formed by kettleholes. But a northern barrier at this point is not evident, 
although a tendency to a northward extension seems to be indicated. . 
Having thus invaded the area of the drift, it is not astonishing that this species 
spread oyer large tracts of the latter in Ohio, Indiana, and Illinois. Its presence to 
the south of the drift in the Mississippi Valley would then be a continuation of this 
westward migration, which finally varied toward the southwest and the south. I 
have represented it as such (Ortmann, 1905d, p. 123, Pl. 3)in a previous paper. Nev- 
ertheless this question needs further investigation. 
There remains the eastern area of this species on the Atlantic Coastal Plain. In 
the paper just referred to I have expressed the opinion (/. ¢., p. 123) that it ‘“de- 
scended from the mountains” toward the east, but this apparently needs correction. 
Of course the direct way from its supposed center to the Atlantic plain is from south- 
western Pennsylvania and northern West Virginia across the Appalachian system 
and the Piedmont Plateau to Maryland and Virginia. But the total absence of this 
species from this region is against this assumption. There is no possible reason why 
it should have disappeared from the Potomac valley, if it had once been present 
there, favorable localities being abundant. 
Comparing, however, the present eastern range of C. diogenes with that of C. 
limosus, we are struck at once by the general similarity. Both species belong to 
the Coastal Plain from New Jersey southward, C. diogenes going a little further 
south, and not quite so far north, while C. limosus has entered the Piedmont 
Plateau, and C. diogenes has not. This similarity induces us to assume a similar 
