BULLETIN FLORIDA STATE MUSEUM


does today. Certain parts of the Mississippi could have once acted
as "filter bridges" for their movement.
 Lowered ocean levels during the glacial periods permitted many
freshwater streams to join before reaching salt water, for example
those draining into the Gulf of Mexico from the Alabama system west
to the Mississippi. During one of the glacial advances some chryso-
cephalus stock in the Mississippi Valley apparently moved through
the lower reaches of the Mississippi into some of the Gulf coastal
streams. When, with the retreat of the glacier, the sea level rose
and restored the Mississippi to its former proportions, the chryso-
cephalus stock isolated in the coastal streams evolved into a new form,
isolepis, here regarded as a well-defined subspecies of chrysocephalus.
One can only speculate when the original invasion of the coastal
streams by chrysocephalus occurred, but the estimated time neces-
sary for a new form of this complex to evolve suggests it was probably
during one of the first two glacial periods. Much later, after the
morphological separation of chrysocephalus and isolepis was com-
plete, the two forms came together again when the Alabama-Coosa
system captured certain tributaries of the Tennessee. Van der Schalie
(1945: 357-358) discusses the geological and malacological evidence
showing the relationships of these two systems.
 Notropis cornutus either survived in the upper Ohio Valley
throughout the Pleistocene or reached there during one of the inter-
glacial periods. Absence of this species from the upper Kanawha
River system suggests a fairly recent invasion, either from head\\ater
capture or through overflow streams from ponded lakes formed by
the advancing Wisconsin glacier. The further advance of the Wis-
consin ice sheet (Flint, 1947: 283) isolated the Ohio Valley popula-
tions of cornutus from both the Atlantic coast and the western popu-
lations.
 As the Wisconsin ice sheet retreated, meltwater lakes formed at
the glacial margin. Because the glacier prevented northward flow,
the rising lakes began to cut outlets to the south. One of the largest,
Lake Agassiz, covered an extensive area and drained into the Mis-
sissippi Valley by means of the Warren River outlet (Radforth, 1944:
13), near the present southern extremity of the Red River of the
North. As the glacier retreated farther, a channel finally opened
to the northeast, the origin of the Nelson River. This stream was
situated at a lower elevation than the older Warren River, and thus
formed a new outlet for Lake Agassiz, which consequently decreased


Vol. 8