Marsh—North American Species of Diaptomus . 391 
this type ; the more nearly equal in length the right and left 
feet, the more primitive the form; the more nearly the endo- 
podite approaches a three-segmented structure of the same 
length as the exopodite, the more primitive the form, etc. Of 
course, this reduction may not have been correlated in different 
structures ; for example, a two-segmented endopodite might pos¬ 
sibly be found in a recent form, although in most a one-seg- 
mented condition exists. Generally ^speaking, however, it would' 
be true that a species would be considered the more primitive in 
proportion as it approaches more or less closely to a form hav¬ 
ing its feet composed of three-segmented rami. It is to be sup¬ 
posed that this reduction may have gone on at the same time 
in two or more lines, so that the fact of a similar stage of re¬ 
duction in the segments of the feet would not in all cases imply 
close relationship, although many times this would be true. 
The common presence in two or more forms of a structure that 
was developed rather than leffloehind in the process of reduc- 
tion 2 would be pretty good evidence of close relationship. 
It must be borne in mind, too, that very little is known of 
the amount of variation in the structures of Diaptomi, and 
further knowledge may modify present conclusions. For ex¬ 
ample, it is difficult to correlate the peculiar armature of the 
antepenultimate segment of the right male antenna with other 
structures, and it is possible that it may appear that these 
structures are much more variable than is now supposed. 
THE OREGONENSIS GROUP. 
This group includes D. oregonensis, Reighardi, mississippi- 
ensis, pallidus, and possibly Bakeri and franciscanus. Leaving 
Bakeri and franciscanus out of consideration for the time be¬ 
ing, the members of the group agree in the following charac¬ 
teristics : 
All are without a distinct appendage on the antepenultimate 
segment of the male right antenna. All have the lateral spine 
near the end of the second segment of the right male exopodite, 
and have a small spine near the inner margin of the same seg¬ 
ment. The terminal processes of the left foot of the male are 
