Among the various more or less tangible species that may be recognized, three distinct series appear. These have been termed the cutthroat-trout series (allies of _Salmo clarkii_), the rainbow-trout series (allies of _Salmo irideus_), and the steelhead series (allies of _Salmo rivularis_, a species more usually but wrongly called _Salmo gairdneri_). The steelhead, or _rivularis_ series, is found in the coastwise streams of California and in the streams of Oregon and Washington, below the great Shoshone Falls of Snake River, and northward in Alaska along the mainland as far as Skaguay. The steelhead-trout reach a large size (10 to 20 pounds). They spend a large part of their life in the sea. In all the true steelheads the head is relatively very short, its length being contained about five times in the distance from tip of snout to base of caudal fin. The scales in the steelhead are always rather small, about 150 in a linear series, and there is no red under the throat. The spots on the dorsal fin are fewer in the steelhead (4 to 6 rows) than in the other American trout. The rainbow forms are chiefly confined to the streams of California and Oregon. In these the scales are large (about 135 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, Washington, and Alaska, and all the clear streams on both sides of the Rocky Mountains, and in the Great Basin and the head-waters 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. _gilberti_. 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 _clarkii_ and _rivularis_, together with a medley of forms apparently intermediate.