ORTMANN: THE CRAWFISHES OF THE STATE OF PENNSYLVANIA 439 
(Indiana) at a time when this basin was entirely covered by ice further east, thus 
being closed more or less to an immigration from the central parts of Ohio (drain- 
age of middle Ohio), and being closed entirely to an immigration from eastern 
Ohio and western Pennsylvania (drainage of upper Ohio). 
This explains why ©. propinquus, which survived in southern Indiana, had the 
first chance to spread northward and to enter the future Huron-Erie basin by way 
of Lake Maumee. The subsequent stages of this lake (Lake Whittlesey, Lake Warren, 
etc.), are all direct continuations in time of Lake Maumee, and so it is not astonish- 
ing that C. propinquus, after the final establishment of the St. Lawrence drainage,” 
is found all over this region, not only in the Lake Huron and Lake Erie basins, but 
also farther down, in Lake Ontario and the Lower St. Lawrence drainage. In the 
occupation of this whole region C. propinquus was not interfered with by the other 
forms, since no opportunity was given to C. propinquus sanborni and C. obscwrus, to 
enter the Erie basin, the drainages of the middle and upper Ohio remaining perma- 
nently changed to the southwest, away from Lake Maumee, a condition which ob- 
tains, with very slight changes, up to the present time. 
However, C. propinquus sanborni as well as C. obscurus, have entered the Lake 
Erie drainage. With regard to the first, it may be sufficient to state that it is found 
in Lorain County, Ohio, in rivers and creeks running into the lake, and this is ap- 
parently due to a comparatively recent immigration under similar conditions as in 
the case of C. obscurus in Pennsylvania. The latter species has been discovered by 
the writer in Crawford and Erie Counties, Pa., in streams flowing to Lake Erie, 
associated with the Lake Erie form, C. propinquus. Thus C. obscwrus must have 
crossed the divide between the upper Beaver (Shenango) River and Alleghany 
River (French Creek) on the one side, and Lake Erie (Conneaut and Elk Creeks) on 
the other, and the question is by what means this was accomplished. 
It is only natural that C. obscwrus, surviving during Glacial times in south- 
western Pennsylvania and West Virginia, migrated up the drainage of the upper 
Ohio, chiefly the Beaver and Alleghany Rivers, in Postglacial times, for after 
the end of the Glacial Period this system formed a unit, and no serious barriers 
to the dispersal were, or are, present. Thus it was easy for this species to go up 
52 The change of the westward drainage to an eastward took place toward the end of the Glacial Period, as soon as 
the ice receded far enough to uncover Lake Ontario (Lake Iroquois), thus permitting the water to drain off through the 
Mohawk, and later through the St. Lawrence. This was accompanied probably by a depression of the land in the 
Northeast, culminating in the marine invasion of theSt J.awrence valley (Champlain submergence). (See Grabau, 1901, 
p. 59 et seq.) 
53 As to the formation of the present Alleghany out of the former Lower, Middle, and Upper Alleghany, see Lev- 
erett, 1902, p. 129 ef seq. 
