388 Wisconsin Academy of Sciences. Arts, and Letters. 



D. Tryhomi, Oregon. 



D. Tyrelli, California. 



D. Wardi, Washington. 



D. washingtonensis, 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 r.s 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, clavipes, Eiseni, franciscanus, Judayi, 

 Lintoni, novamexicamis, n/adus, sdltillinus, sTioshone^ signi- 

 camda, Tryhomi, Tyrelli, 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 thai 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. 



RELATIOK OF STRUCT UEE 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 with 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 



