Marsh — North American Species of Diaptomus. 397 



D. Lintoni has been found only in the Yellowstone park. D. 

 conipedatus has thus far been found only in Louisiana, and 

 D. clavipes in Iowa and Nebraska. D. spatulocrenatus has 

 been found only in the island of Nantucket. D. stagnalis prob- 

 ably occurs generally throughout the Mississippi valley, and D. 

 leptopus not only in the Mississippi valley but north into Brit- 

 ish America. , , 



I have called this the leptopus group because D. leptopus ia 

 the most common species. In many respects, however, D. stag- 

 nalis may be considered the more primitive. This is notably 

 the case in the characters of the female fifth feet — the three- 

 segmented exopodite, and the two-segmented endopodite. From 

 its distribution, one would think of D. leptopus as the more an- 

 cient form. In the antennal armature, D. stagnalis, D. coni- 

 pedatus, D. spatulocrenatus and D. Lintoni are most nearly 

 alike. 



I think one can do little more than guess at the relationships 

 of the members of the group. D. clavipes is apparently the 

 most specialized. The probable relationships may be put, pro- 

 visionally, as follows: 



clavipes 



si>atulocrenatus 



Lintoni 



conipedatus 



leptopus I stagnalis 

 THE SIGNICAUDA GROUP. 



This consists of D. signicauda, ivashinjtonensis, nudus and 

 Judayi. They are put together because of the common character 

 of the posterior process on the female abdomen. D. Judayi has 

 a straight process on the antepenultimate segment of the right 

 antenna of the male. The others agree in having a hook. D. 



