392 Wisconsin Academy of Sciences, Arts , and Letters. 
digitiform in D. oregonensis, franciscanus and Reighardi, the 
inner one is falciform in D. pallidus, while in D. mississippi- 
ensis and D. Baheri they are elongated and digitiform. In the 
female the fifth feet are nearly the same in all these species. 
All have two spines on the second segment of the exopodite. 
The lateral spines of the female abdomen are most pronounced 
in D. Baheri and D. mississippiensis, but are not large in any 
of the species. In D. oregonensis the right and left feet are 
nearly of the same length. In the other cases there is consid¬ 
erable difference between the lengths of the two feet. 
As to distribution, D. oregonensis is known to live from 
about the parallel of 42 degrees north to the Saskatchewan re¬ 
gion, and very likely has a still further extension towards the 
Arctic. D. Reighardi is localized, so far as is known, to the 
northern part of the southern peninsula of Michigan. D. mis¬ 
sissippiensis is found only in the gulf states. D. pallidus is 
found in the Mississippi valley from Wisconsin south, and as 
far west as the Rocky mountains. D. Baheri and D. frcmcis- 
canus have been found only in California, and each only in a 
single locality, although, of course, it is probable that they will 
be found to have a wider distribution. 
It seems to me that D. oregonensis, pallidus, Reighardi and 
mississippiensis have clearly a very close relationship, as shown 
by the absence of the antennal appendage and the very close 
resemblance in the male fifth feet; this is noticeable in the pro¬ 
portions of the segments, the location of the lateral spine, and 
especially in the presence of the small spine near the inner 
margin. D. Baheri and D. franciscanus have this small spine, 
which makes me feel quite certain that they must be in the 
same line. 
The phvlogeny of part of the group seems to me quite clear. 
D. oregonensis is the most primitive form. Its wide distribu¬ 
tion in the cold temperate region is entirely in harmony with 
this supposition. It or its immediate ancestor inhabited the 
waters of the United States south of the ice at the height of 
the glacial period. As the ice disappeared, it gradually moved 
towards the north, adapting itself only in a slight degree to 
