GILBERT: FISHES OF THE SUBGENUS LUXILUS


the warmer conditions and largely dispossessed it in the southern
part of its range. As these species occupy similar ecological niches,
competition apparently extirpated cornutus from most of the Ohio
Valley.
 The presence of isolated populations of cornutus in the middle
part of the White River system in south-central Indiana and in the
lower Kanawha River system of West Virginia strongly supports this
view. Although these populations may have been introduced by
man, the evidence strongly favors their presence as natural. Though
populations of chrysocephalus live only short distances away (see
maps 4 and 5), there is no evidence that it has ever occurred in these
pockets occupied by cornutus. I collected in these areas in August
and September 1958, some 25 years after the cornutus colonies were
discovered, and failed to find chrysocephalus. Apparently some bar-
rier has prevented its moving in here as it has elsewhere.
 In the Indiana population the barrier is almost certainly eco-
logical. Mill Creek is unusual in having a shifting-sand bottom
almost throughout its length, with short stretches of rubble and gravel
riffle in a few places where sand has not covered the bottom. N.
cornutus undoubtedly spawns in these limited stretches and has been
able to survive by this thin margin. Apparently the stream is fed by
a number of small springs, for on 26 August 1958 the water tempera-
ture in a shallow, unshaded section of the stream was only 700 F.,
much lower than might be expected considering the weather at the
time. This stream is also one of the few places in southern Indiana
where Rhinichthys atratulus can still be found. The habitat in Mill
Creek is marginal for cornutus and has not allowed the population to
reach large numbers. Paradoxically it has also permitted the species
to survive, for the long stretches of shifting sand bottom have appar-
ently effectively barred the movement of chrysocephalus into the
area.
 The reasons for chrysocephalus' absence from Paint Creek in the
lower Kanawha system are less obvious. The area where cornutus
was collected appears suitable for chrysocephalus, which has been
collected near the mouth of Paint Creek. Thus some barrier, either
physical or ecological, must exist in the lower half of the stream. In
its lower reaches Paint Creek is a gravel-bottomed stream consisting
almost entirely of shallow riffles with few or no deep pools. Perhaps
this habitat is unsuitable for chrysocephalus.
 LIFE HISTORY AND ECOLOGY. N. cornutus prefers small to medium-
sized streams with clear, cool, weedless water, a moderate to swift
current, and alternating pools and riffles, the latter with a gravel or