Marsh—North American Species of Diaptomus . 393 
the changes of the environment. D. pallidus may he derived 
from D. oregonensis . It has gradually taken the place of D. 
oregonensis through the Mississippi valley. The difference in 
structure between D. oregonensis and D. pallidus is doubtless 
due to minor influences of the environment, accompanied by 
isolation. It is not likely that the morphological differences 
can be traced to any specific influences of environment. D. 
mississippiensis, which is still further removed from I), oregon¬ 
ensis, is of later origin, and in a more limited habitat has de¬ 
veloped more pronounced structural differences. 
D. Baheri and D. franciscanus differ from the other members 
of the group in that they have an appendage on the antepenul¬ 
timate segment of the right male antenna. If it should appear 
that this appendage is an invariable character, these two species 
should, doubtless, be separated from the group. The position 
of the lateral spine on the right exopod ite of the male fifth 
foot and the presence of the small inner spine on the same seg¬ 
ment make one feel that these two species must belong in the 
oregonensis group. 
The endopodites of the male fifth feet in D. Baheri are tw T o* 
segmented, and the right exopodite in D. franciscanus is fre¬ 
quently two-segmented. The endopodites of the female fifth 
feet in D. Baheri are also two-segmented, and in both D. Baheri 
and D. franciscanus the exopodites of the female fifth feet are 
three-segmented. Thus D. Baheri and D. franciscanus have 
many of the characteristics of what I have assumed to be prim¬ 
itive structures, and it is possible that these peculiarities have 
been retained in their somewhat limited habitat; this must be 
a matter of conjecture, however, because of our slight knowl¬ 
edge of the distribution of the California species. 
The relationships of the members of the group may then be 
expressed in the following way: 
