394 Wisconsin Academy of Sciences, Arts, cmd Letters. 
mississippiensis 
THE TENUICAUDATUS GROUP. 
This group includes D. tenuicaudatus, sicilis, Ashlandi, mi- 
nutus, Birgei, siciloides, Wardi 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 species 
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- 
