610 On the Distribution of Fresh- Water Fishes.  [October, 
In the genus Nocomis ( Ceratichthys Baird) N. biguttatus probably 
occurs in every river from Pennsylvania to the Great Salt Lake, 
while four species of the same genus, N. micropogon, N. monachus, 
N. zanemus, and N. labrosus, are each,so far as is known, confined 
to a single river basin. 
XI. In any river basin the most abundant species (of small 
fishes) are usually (a) those peculiar to it, or some of them; or 
(6) those of the widest distribution. In illustration of this we 
may notice the abundance of “ Photogenis”’ pyrrhomelas and No- 
totropis photogenis in the Santee; of ** Photogenis” stigmaturus 
and Luvilus cornutus in the Alabama; of ‘+ Photogenis” eurys- 
tomus and Nocomis biguttatus in the Chattahoochee; of “ Pho- 
togenis” xenurus and Notemigonus Americanus in the Ocmulgee. 
To this rule, however, there are many exceptions and modifica- 
tions. 
XII. In general, the further south any river basin lies, the 
more species are peculiar to it and the greater the difference be- 
tween its fauna and that of the neighboring streams. In illus- 
tration of this, the differences existing between the faune of the 
Alabama and Chattahoochee may be compared with those be- 
tween the Susquehanna and the Delaware. Twelve genera arè 
known as common to the Alabama and Chattahoochee, twenty- 
three to the Susquehanna and Delaware. In the Southern streams, 
the process of evolution of specific forms seems to have progre 
more rapidly. i 
XIII. Species of the widest distribution often have breaks in 
their range which cannot be accounted for by any facts now mM 
our possession, Luzilus cornutus, — the common shiner or red-fin 
of New England, — so abundant in all the rivers of the North and 
West, does not occur, so far as is known, in any of the rivers be- 
tween the Neuse and the Alabama, in both of which streams 1t 18 
very numerous. Various other species range over several river 
basins and then cease abruptly. Amuirus brunneus is the most 
abundant food-fish in the rivers from the Santee to the Chatta- 
hoochee, while in the next river westward — the Alabama — it 
is unknown. 
XIV. Many species of wide distribution which are pees ide 
certain streams are there represented by certain other . 
species which may be regarded as modified descendants. Thus, 1” 
the South Atlantic streams, Chenobryttus gulosus is represen’ 
by Chenobryttus viridis, Notemigonus chrysoleucus by Notemig? 
nus Americanus, etc. In the Southwest, Hupomotis auren? 18 
, 
+ 
