72 SCIENCE SKETCHES. 
trout, we find no differences, other than that the 
former is of much larger size, and has a larger 
mouth, and its caudal fin is truncate instead of 
forked. But the tail becomes more truncate, and 
the mouth larger with age in all species of salmon 
and trout. If a rainbow trout were to reach the 
size of the steel-head, it ought to acquire charac- 
ters similar to those of the latter species. Con- 
versely, unless the rainbow trout are young of the 
steel-head, the young of the latter are unknown. 
It is my belief that the steel-head is simply the 
large rainbow trout which has lived in the sea, and 
ascends the river to spawn. If this be true, Salmo 
tvideus must be omitted from our lists, as identical 
with Salmo gairdnerz, the latter name being the 
earlier one. 
The most widely distributed, and decidedly the 
most important, of the American black-spotted 
trout is the Salmo mykiss (= Salmo purpuratus 
and clarki of authors), or, as we call it, the Red- 
throated Trout. This species has much smaller 
scales than the rainbow trout or steel-head, the 
usual number in a longitudinal series being 150 
to 170. Its mouth is proportionately larger, and 
there is usually a narrow band of small teeth on 
the hyoid bone at the base of the tongue. 
These teeth are always wanting in Salmo irideus 
and gatrdnert. The color in Salmo mykiss is, as 
in other species, excessively variable. In almost 
all specimens there is a deep-red blotch on the 
throat, between the branches of the lower jaw and 
the membrane connecting them. This I have not 
found in other species; and as it seems to be 
