392 Wisconsin Academy of Sciences, Arts, and Letters. 



digitifonn in D. oregonensis, franciscanus and Reighardi, the 

 iimer one is falciforai in D. pallidus, while in D. mississippi- 

 ensis and D. Bakeri they are elongated and digitifonn. 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. Bakeri and D. mississippiensis, hnt 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- 

 sissippie7isis 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. Bakeri and D. francis- 

 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. Bakeri and D. francisconns have this small spine, 

 which makes me feel quite certain that they must be in the 

 same line. 



The phylogeny of part of the group seems to me quite clear. 

 D. oregonensis is the most primitive form. Tts 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 uortli, adapting itself only in a slight degree to 



