Marsh — North American Species of Diaptomus. 393 



the changes of the environment. D. pallidus may be 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. T). 

 mississippiensis, which is still further ivmoved from D. oregon- 

 ensis, is of later origin, and in a more limited habitat has de- 

 veloped more pronounced structural differences. 



D. Bakeri 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 exopodite 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. Bakeri are two- 

 segmented, and the right exopodite in D. franciscanus is fre- 

 quently two-seginented. The endopodites of the female fifth 

 feet in D. Bakeri are also two-segmented, and in both D. Bakeri 

 and D. franciscanus the exopodites of the female fifth feet are 

 three-segmented. Thus D. Bakeri 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 somew^hat 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: 



