GILBERT: FISHES OF THE SUBGENUS LUXILUS


zonatus and Notropis pilsbryi. The cause of this isolation also is un-
known, although it probably is related to the fact that the Ozarks
contain clear, cool, rather swift-flowing streams. Unlike some other
Ozarkian endemics, Notropis zonatus and Notropis pilsbryi have no
immediate relatives living in upland areas east of the Mississippi
River. Consequently these species are believed to be autochthonous
for the Ozark region.
 The distributions of N. zonatus and N. pilsbryi suggest a long sep-
aration between the Missouri and White river systems that has pre-
vented gene interchange. This interpretation is supported by the
parallel distribution patterns of other species pairs, such as Etheo-
stoma euzonum-E. tetrazonum. The presence of N. zonatus in the
Black River system strongly suggests that this river at one time was
tributary to the Missouri, and that a reversal of flow has occurred
rather recently, long after zonatus and pilsbryi separated. Records
of zonatus from the headwaters of the St. Francis and Little rivers
of southeastern Missouri are best explained on the basis of stream
captures, although the possibility exists that these streams also once
flowed to the north. Probably the most perplexing aspect of the
zoogeography of these species has to do with the presence of pilsbryi
in the Arkansas and Red river drainages. The distribution of sev-
eral species (Notropis spilopterus and Lepomis megalotis) indicates a
definite faunal relationship between the Illinois and Neosho rivers
(tributaries of the Arkansas) and the Ozarkian tributaries of the Mis-
souri River. The absence of Notropis zonatus from the Illinois and
Neosho is an apparent contradiction, inexplicable save for the possi-
bility that if zonatus did invade these river systems it could not com-
pete with the well-established population of pilsbryi. Hubbs and
Moore (1940: 94) have suggested the isolated populations of pilsbryi
in the Red River system may have been introduced. It is equally
possible that they represent relict populations, especially as they have
been recorded from three separate localities in this drainage.
 Prior to the Pleistocene epoch the drainage pattern of the Ohio
Valley differed considerably from that of today. The Ohio River
extended little east of Cincinnati, and what is now the upper Ohio
flowed northward, probably to a river in the area of the present Lake
Erie basin. Larger than either of these streams was the Teays River,
a major prolongation of the Kanawha River. The Teays flowed north-
westward across Ohio, Indiana, and apparently Illinois, and thence
southward to the present Mississippi Valley. It occupied a wide, deep
trench comparable in size with that of the lower Ohio River today
(Flint, 1947: 166).