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 plight pos- 

 sibly be found in a recent form, although in most a one-seg- 

 mented condition €ixists. 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 left^ehind in the process of reduc- 

 tioUj 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 diificult 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, Beighardi, mississippi- 

 ensis, pallidus, and possibly Bdken and franciscanus. Leaving 

 Bakeri and franciscanus out of consideration for the time be- 

 ing, the members of the group a^ee 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 



