Marsh—Neiv Species of Diaptomus from Colorado. 199 
This species is closely connected with Tyrelli and was at first 
thought to be identical with it. The differences, however, are 
constant and are of sufficient importance to justify a specific 
separation. The principal points of difference are as follows: 
In the female fifth foot, the spines of the first basal segments 
are large instead of small as in Tyrelli. The endopodite is 
shorter than the first segment of the exopodite, while in Tyrelli 
it is longer. In the male fifth foot the distal margin of the first 
segment of the right exopodite has two hyaline processes, the 
one at the inner angle being quite large, while Tyrelli has one 
inconspicuous process on the inner distal angle. The lateral 
spine of the second segment of the right exopodite is rather 
strongly curved instead of nearly straight as in Tyrelli, and the 
blunt spine of the dorsal surface of this segment I have never 
seen in Tyrelli. The right endopodite is nearly as long as the 
first segment of the exopodite while in Tyrelli it is much 
shorter. The left endopodite of the male fifth foot is much 
stouter than in Tyrelli. 
D. coloradensis is interesting as indicating a possibility of 
bridging over between the oregonensis and tenuicaudatus groups. 
In the Revision of the Rorth American Species of Diaptorrtm 
the author stated*' that Tyrelli apparently was most closely re¬ 
lated to the tenuicaudatus group although not having any ap¬ 
pendage of the antepenultimate segment of the right male 
antenna as in that group. The presence of the small blunt 
spine on the dorsal surface of the second segment of the exopo- 
dite of the male fifth foot in coloradensis is evidence of relation¬ 
ship to the oregonensis group. While, in the light of our 
present knowledge we can only make a guess at the phylogeny 
of Tyrelli and coloradensis it seems probable that they should 
be classed with the oregonensis group. 
* Trans. Wis. Acad. Sci., Letters, and Arts, XV, p. 396. 
