388 Wisconsin Academy of Sciences, Arts, and Letters. 
D. Trybomi, Oregon. 
D. Tyrelli, California. 
D. Wardi, Washington. 
D w ashing tonensis, Washington. 
It will be noticed that some are restricted pretty closely to 
the Mississippi valley, some to the gulf states, and others to 
the northern tier of states. Generally speaking, within rather 
wide limits, the distribution is one of latitude. The most 
northern species, D. minutus, ranges from Iceland to southern 
Wisconsin, D. oregonensis has the same southern limit and has 
been found as far north as the Saskatchewan, D. pallidus ranges 
from Wisconsin to the Gulf, while D. mississippiensis is con¬ 
fined to the gulf states. Strictly confined to the western part 
of the United States and mostly in the mountain regions, are 
D. albuquerquensis, clampes, Eiseni, franciscanus, Judayi, 
Lintoni, novamexicanus, nudus, saltillinus, shoshone, signi - 
ccmda, Trybomi, Tyrelli f Wardi and washingtonensis. 
The greater number of species in the West is doubtless 
largely accounted for by the results of isolation, but it should 
be remembered that collections have been made in only a few 
localities, and it may be found both that the known species 
have a greater range than now appears, and that intermediate 
forms may come to light which will affect present species lim¬ 
its. 
RELATION OF STRUCTURE TO HABITAT. 
Except in a very general way, it is difficult to correlate habi¬ 
tat and structure. Still, certain facts are evident. 
1. Peculiar, bizarre characters are more apt to appear in 
animals living in shallow waters and with a narrow range of 
habitat. This appears in the dorsal process of D. dorsalis, and 
in the hook on the fifth foot of the male in D. clavipes. The 
process of the first abdominal segment of the female is found 
only in D. signicauda and the species associated v T ith it, and 
these are limited to the mountain regions. This principle, 
however, does not apply as widely as we should expect. 
2. There is a marked distinction between species living in 
deep water, and in shallow. The deepwater, or limnetic, forms 
