282 Wisconsin Academy of Sciences, Arts, and Letters. 
Fourth foot 
Inner hr. 
ex. 1 seta- 
ap. 2 sp. 
in. 2 setae. 
Fifth foot two-jointed) basal joint very broad, armed with one 
seta. Terminal joint armed with a seta and a blunt spine. 
This takes the place in our fauna that is occupied by C. viridis 
Fischer in Europe. In general form and appearance the two 
forms seem identical, and have been so considered by Herrick 
and Cragin. I have hesitated to propound a new species name, 
but it seems necessary. So far is Uljanin and Vosseler have 
figured viridis it corresponds to our species; but neither gives 
figures of the swimming feet. From the original description 
by Fischer our species differs markedly. * * He gives a 
figure of a foot, not designating which, but it corresponds to no 
one of the four in our species. * * In an examination of 
a large number of specimens from widely separated localities 
I have found no variation in the number and arrangement of 
the spines and setae of americanus, and until such variation is 
shown, there seems to be no alternative but to institute a new 
species for the American form.” 
These: statements show that investigators, with reference to 
the species of the genus Cyclops in general, regard the arma¬ 
ture of the swimming feet and the form and measurements of 
the furca and its apical bristles, as of varying, but usually of 
secondary importance for specific distinction. The variability 
in these features seems to be considered as lessening their dis¬ 
tinctive value. However, if definite limits of variation can be 
established for a variable feature, it seems reasonable that it 
might prove as valuable for specific distinction as one showing 
almost no variation. 
With reference to the particular species C. brevispinosus both 
Herrick and Marsh consider the form of the caudal stylets and 
their outer apical spines as important for distinction, and 
Marsh gives special importance to the armature of the swim¬ 
ming feet. Marsh also considers the armature of the 4th 
