Salmonidz 97 
The rainbow forms are chiefly confined to the streams of 
California and Oregon. In these the scales are large (about 13 
in a lengthwise series) and the head is relatively large, forming 
nearly one-fourth of the length to base of caudal. These enter 
the sea only when in the small coastwise streams. Usually 
they have no red under the throat. The cutthroat forms are 
found from Humboldt Bay northward as far as Sitka, in 
the coastwise streams of northern California, Oregon, Wash- 
ington, and Alaska, and all the clear streams on both sides 
of the Rocky Mountains, and in the Great Basin and the 
headwaters of the Colorado. The cutthroat-trout have the 
scales small, about 180, and there is always a bright dash of 
orange-red on each side concealed beneath the branches of the 
lower jaw. Along the western slope of the Sierra Nevada there 
are also forms of trout with the general appearance of rainbow- 
trout and evidently belonging to that species, but with scales 
intermediate in number (in McCloud River), var. shasta, or with 
scales as small as in the typical cutthroat (Kern River), var. 
gilbertt. In these small-scaled forms more or less red appears 
below the lower jaw, and they are doubtless what they appear to 
be, really intermediate between clarkii and irideus, although 
certainly nearest the latter. A similar series of forms occurs in 
the Columbia basin, the upper Snake being inhabited by clarkii 
and the lower Snake by clarki and rivularis, together with a 
medley of forms apparently intermediate. 
It seems probable that the American trout originated in 
Asia, extended its range to southeast Alaska, thence southward 
to the Fraser and Columbia, thence to the Yellowstone and 
the Missouri via Two-Ocean Pass; from the Snake River to the 
Great Basins of Utah and Nevada; from the Missouri south- 
ward to the Platte and the Arkansas, thence from the Platte 
to the Rio Grande and the Colorado, and then from Oregon 
southward coastwise and along the Sierras to northern Mexico, 
thence northward and coastwise, the sea-running forms passing 
from stream to stream. 
Of the American species the rainbow trout of California 
(Salmo irideus) most nearly approaches the European Salmo 
fario. It has the scales comparatively large, although rather 
smaller than in Salmo jfario, the usual number in a longitudinal 
II—7 
