394 Wisccmsin Academy of Sciences, Arts, amd Letters. 



mississippiensis 



Bakeri pallidus 



frauciscanus | I — Reighardi 



oregonensis 



THE TENUICAUDATUS GROUP. 



This group includes D. tenuicaudatus, sicilis, Ashlandi, mi- 

 nutus, Birgei, siciloides, Wa/rdn, and shoshone. All, with the 

 exception of Birgei and siciloides, agree in having a slender 

 straight appendage on the antepenultimate segment of the right 

 antenna of the male; these, however, are not the only speciee 

 with this appendage. The male fifth feet of D. tenuicaudatus, 

 D. sicilis and D. shoshone resemble each other very closely. In 

 D. Ashlandi and D. Birgei the most marked difference is in 

 the position of the lateral spine. 



D. Birgei has an exceedingly short appendage on the ante- 

 penultimate segment of the male right antenna, and D. sici- 

 loides has a short hook. In all members of the group, with the 

 exception of D. shoshone and D. minutus, there is a hyaline 

 lamella on the posterior surface of the first segment of the 

 right exopodite. This peculiarity is found in some species 

 outside this group, especially in those of the signicauda group, 

 and may indicate a common structural relationship between the 

 groups. D. minutus differs not only in the position of the 

 lateral spine, but in the form of the terminal hook, and in the 

 rudimentary right endopodite. In the female fifth foot, too, 

 D. minutus differs in that the endopodites are rudimentary, 

 and that they have three spines on the second segment of the 

 exopodite. D. shoshone has elongated terminal spines on the 

 endopodite of the female fifth foot and has a distinctly three- 



