GILBERT: FISHES OF THE SUBGENUS LUXILUS


from the Chickasawhay drainage in Mississippi show 14 with 9 anal
rays and 17 with 10.
 Sexual variation is pronounced in N. c. isolepis, as in the closely
related N. cornutus and N. c. chrysocephalus. Differences presumably
are identical to those occurring in the latter form and very similar to
those in the former. Breeding males develop a deep red coloration
on body and fins, the black crescent-shaped markings on the side
of the body become notably darker and more pronounced, and prom-
inent nuptial tubercles develop on the head, back, anterior part of the
dorsal and upper surface of the pectoral fins. Females have a more
subdued coloration and usually lack tubercles. Adult males also de-
velop a deeper and more compressed head, have slightly longer pec-
toral and pelvic fins, and an increased amount of adipose tissue sur-
rounding the fin rays, but apparently do not differ in other respects
from the females (table 6).
 RELATIONSHIPS. N. c. chrysocephalus is the only form of Luxilus
that occurs with N. c. isolepis in the Gulf drainage. Both are found
in the Coosa River system and, although they occur in adjacent tribu-
tary streams a few miles apart, they have not yet been found to-
gether. Specimens from one collection taken in an eastern tributary
of the neighboring Black Warrior system have been identified as in-
tergrades, and several other collections in the same drainage have
been identified as N. c. chrysocephalus. Perhaps transfer of chryso-
cephalus into the Black Warrior has been effected through a localized
stream capture. The apparent lack of intergradation of chryso-
cephalus and isolepis in the Coosa basin is difficult to explain, though
this may in some way be related to the fact that, in this area at least,
the ranges of the two forms appear to be separated by the so-called
Fall Line which marks the transition from an upland to a lowland
environment (Fenneman, 1938: 126-131). On the other hand these
forms may be incipient species which, because of ecological similari-
ties, have each prevented the other from moving into the areas it
occupies. Should this explanation be correct the intergradation in
the Black Warrior drainage would be difficult to explain.
 DISTIBUTION. Confined to tributaries of the Gulf of Mexico from
the eastern half of the Red River system (including the Ouachita
River) in Texas, Louisiana, Arkansas and Oklahoma, eastward to the
Tallapoosa drainage in western Georgia (UF 9563).
 Its range west of the Mississippi River is, with one exception, re-
stricted to the. Red River system. A single record from the Arkansas
drainage in Arkansas (USNM 165878) is interpreted as the result of
either human introduction or localized stream capture. This form


1964