Birge—Notes on Cladocera. 
1047 
rate, the species differs widely from the American form, which 
I have identified with A. tenuicaudis and which occurs in 
Lake Charles, as well as elsewhere in the United States. I 
give figures of the post-abdomen of a specimen taken from 
the same collection as the Odontalona figured. (PI. LXXI, 
figs. 5, 6.) The differences are obvious. 0. tenuicaudis has 
a cluster of 3 or 4 long, marginal denticles at the apex of the 
post-abdomen, and all the rest are very small. 0. longicaudis 
has but two large denticles and those of the rest of the row are 
good sized. The post-abdomen is much longer and narrower 
than in 0. tenuicaudis, although that species differs consider¬ 
ably in this respect. In Lilljeborg’s figure (’00, PI. LXVIII, 
figs. 4, 5) it is much longer than in that of P. E. Mueller (’68, 
PI. Ill, fig. 2) and this in turn is longer than in the figure of 
Matile (’90, PI. IV, fig. 33). All of our specimens in America 
seem to be nearer to the form shown by Matile than to any of 
the others, although shorter and broader than any figured by 
European authors. The post-abdomen of northern specimens is 
longer than those from the south. In all cases the form and 
attachment of the basal spine of the terminal claw are char¬ 
acteristic for the species (PI. LXXI, fig. 6), and in this re¬ 
spect 0. tenuicaudis differs from all other Chydorina. The 
structure of this region in 0. longicaudis is also unique but 
decidedly different from 0. tenuicaudis. The labrum of 0. 
tenuicaudis has the regular Alona form, while in 0. longicaudis 
it is more nearly angled behind. 
It is not easy to settle the generic relations of this species. 
It is certainly very close to 0. tenuicaudis and that species 
must go into the same genus. Xo one before Daday thought 
of separating 0. tenuicaudis from Alona. The color and gen¬ 
eral habit of the new form strongly recall Euryalona, and I 
assigned it at first to that genus without hesitation. This is 
the opinion of Daday. Yet careful study convinced me that 
this position could not be given to it; since it lacks almost 
every character used by Sars to characterize the genus Eury¬ 
alona (’01, p. 80). The head is not noticeably small; the 
valves are not tumid and gaping in front; the setae are not 
restricted to the middle of the ventral margin; the first foot 
