GILBERT: FISHES OF THE SUBGENUS LUXILUS


the extremes 22 and 33; a distribution expressed as (8) 9 or 10,
usually 9, shows that less than 10 percent of the counts number 8
and, while a count of 10 is not particularly rare (more than 10 percent)
9 is a much more common value.
 To determine possible sexual variation, approximately equal num-
 bers of males and females were measured for the various morpho-
 metric characters. In most cases the means and range of variation
 differed so little and the overlap was so great that measurements were
 lumped in the final tabulations. Tables and lists of locality records
 in the original dissertation not regarded as essential have been omit-
 ted. Only new names or new combinations of names have been re-
 tained in the synonymy.

 HYBRIDIZATION
 Numerous hybrid combinations involving members of the Luxilus
complex are known. Three of these, N. c. chrysocephalus X N. cor-
nutus, N. c. chrysocephalus X N. rubellus, and N. cornutus X N.
rubellus, are the most common cyprinid hybrids in eastern North
America. The cornutus X rubellus hybrid was described by Jordan
(1889a: 354) as a new species, Notropis macdonaldi, and the chryso-
cephalus X rubellus cross was erroneously recorded by Forbes (1909:
386) and by Forbes and Richardson (1920: 149) as Notropis pilsbryi.
 A number of papers have appeared dealing entirely or in part
with various aspects of hybridization in certain species of Luxilus.
Among the more comprehensive and important of these are Raney's
(1940b) account of the comparative spawning behavior of N. cornutus
and N. rubellus in New York, and Miller's (1962; 1963) papers deal-
ing with the comparative morphology, sexual development, and
hermaphroditism of N. cornutus X N. rubellus. In my own (1961b)
treatment of the relationships of N. cornutus and N. c. chrysocephalus
I list the arguments for and against specific differentiation of cornutus
and chrysocephalus, and favor their recognition as species, although
the two forms often act as subspecies. This evidence is as follows:

 1. Intergrading populations of subspecies normally show a per-
fect blending of morphological characters. Although this is ap-
proached in some associations containing both cornutus and chryso-
cephalus, in other populations the two forms remain morphologically
distinct, with few apparent intermediates. In addition many popu-
lations within the zone of "intergradation" contain only one form.


1964