GILBERT: FISHES OF THE SUBGENUS LUXILUS


rapidly in area and was superseded by Lake of the Woods, Lake Win-
nipeg, and many lesser lakes.
 Soon after formation of Lake Agassiz, there appeared farther to
 the east glacial lakes Duluth, Chicago, and Maumee, the precursors,
 respectively, of lakes Superior, Michigan, and Erie. These bodies,
 also plugged at their northern extremities, drained into the Missis-
 sippi Valley by way of the St. Croix outlet (Lake Duluth), the Fox and
 Chicago outlets (Lake Chicago) and the Fort Wayne outlet (Lake
 Maumee). With further glacial retreat changes continued until the
 present-day Great Lakes were eventually formed.
 N. cornutus probably moved northward via the Warren River
 outlet into Lake Agassiz, and reinvaded the newly-formed Great
 Lakes from the south through glacial channels such as the St. Croix
 and Fox rivers (Greene, 1935: 15) and from the north via glacial Lake
 Agassiz through the Ft. William outlet in western Ontario (Underhill,
 1957: 31). Notropis cornutus apparently was able to follow the re-
 treating glacier more closely than many other species and, as a result,
 was again able to invade the east coast, this time via the Horseheads
 outlet (in the Lake Ontario drainage) into the Upper Susquehanna
 system (Bailey, 1945: 125-126). There it again mixed with the popu-
 lation of cornutus which had remained on the Atlantic coast presum-
 ably throughout the Pleistocene.
 At this time ocean levels still were much lower than at present.
The shallow bottom of Chesapeake Bay was above water, and all
streams from the Susquehanna south to and including the James were
interconnected. Although influence of the cornutus stock that crossed
onto the Atlantic coast may have extended into the Chesapeake Bay
area, the bulk of the western gene pool probably remained near the
point of original entry. Proof that the preceding sequence of events
occurred depends in large part on the usual absence of pigmentation
on the chin and gular region in cornutus from the upper Susquehanna.
This character, while not consistent enough to be taxonomically name-
worthy, nevertheless is of value in the determination of zobgeograph-
ical relationships among populations. Pigment usually is lacking
west of the Alleghenies, but not in coastal streams.
 N. cornutus may possibly have utilized a temporary outlet between
the forerunner of Lake Ontario and the Mohawk River. Apparently
some species did enter the Mohawk in this way, but the history is
blurred by the construction of the Mohawk-Erie canal across the
drainage divide in the early 1800's. Notropis cornutus seems to have
moved a considerable distance eastward from its western glacial refu-


1964