GILBERT: FISHES OF THE SUBGENUS LUXILUS


range specimens of cornutus are heavily pigmented not only on the
underside of the head, but over the entire body. This heavy pigmen-
tation, often so intense that individuals are almost black, is without
doubt a direct result of environmental conditions. Many northern
waters are clear but heavily stained or tea-colored, which apparently
causes an expansion and multiplication of melanophores over the
entire body.
 The size of the predorsal scales shows the greatest geographic
variation of any character encountered in this species. Within a
given population scale size is usually more or less uniform. This is
not invariable, and in several series scale size has been found to
vary considerably. There may even be great variation between popu-
lations in adjacent streams. Scale size generally decreases gradually
from south to north, a gradient that is particularly noticeable in
Atlantic coast populations. Northeastern populations, however, show
an abrupt increase in average scale size (a decrease in scale counts),
which reverses the trend seen in other coastal populations of cornutus.
 Individuals on the Atlantic coast have larger anterior dorso-lateral
and circumferential scales, on the average, than those living west of
the Appalachians. In the upper Susquehanna River system scale size
averages somewhat smaller than for other coastal populations, a fur-
ther indication of a post-Wisconsin invasion of the Susquehanna by
western populations of cornutus. Difference in scale size has been
the basis for a subspecific separation of populations from opposite
sides of the Appalachians (Hubbs, 1926: 45-46). The desirability of
maintaining this separation, however, has been questioned by some
authors (Hubbs and Brown, 1929: 39; Greeley, 1938: 67), and their
observations are confirmed by the present study.
 Sexual dimorphism is pronounced in this species, as in most other
nest-building or territorial cyprinids, such as Semotilus atromaculatus,
Campostoma anomalum, and Hybopsis biguttata. Some differences
are not evident until the breeding season, when males develop a
deep red coloration on body and fins, as well as prominent nuptial
tubercles on the head, back, anterior part of the dorsal fin, and upper
surface of the pectoral fin. Females, in contrast, have a more sub-
dued coloration and usually lack tubercles. Other sexual differences
are present throughout the year and become pronounced in males
only during the breeding season. This is particularly true of the
black crescent-shaped markings on the side of the body, which be-
come notably darker and more prominent. Also the mid-dorsal stripe
and the narrow, light, parallel, longitudinal stripes on the lower part
of the back become relatively lighter, contrasting markedly with the