290 Wisconsin Academy of Sciences, Arts, and Letters. 
The small number of specimens examined and the limited 
range of the localities from which they were obtained, must of 
course! prevent, the 1 drawing of absolute conclusions from these 
figures. 
That, the armature of the fourth swimming foot is not con¬ 
stant in C. americamis, is shown in Plates. XXXII and 
XXXIII, distinct spines, distinct setae, and intermediate 
forms being found on the external side of the terminal joint of 
the inner branch. The seta, is the form of armature found 
most frequently, but the- presence of the other 1 forms indicates 
that this cannot be considered a constant feature. 
The variations in the length of the furca in the specimens of 
americanus measured, are wide, the limits being .052 mm. and 
.19 mm., thus including the limits for this character of brevis¬ 
pinosus. The width of the furca is generally greater in amer¬ 
icanus than in brevispinosus and varies less widely than the 
measurements of length, the limits for basal width being .025 
mm. and .0625 mm.; and those for the width at, the terminal 
end .015 mm. and .04 mm., thus also including the limits of 
variation in width of the furca of brevispinosus. The outer 
terminal seta of the furca of americanus is most frequently 
moderately slender, sometimes considerably thickened at, the 
base, and frequently almost impossible to distinguish from the 
so-called spine of brevispinosus. 
The comparative relation of the length of the last two or 
three abdominal segments to the length of the furca is also vari¬ 
able in each species. The furca is in some cases, hardly longer 
than the last abdominal segment, and in others is as long as the 
last three segments. 
The descriptions of Marsh and Herrick for these forms of 
Cyclops make the characters considered in this paper of con¬ 
siderable importance for specific distinction. It seems impos¬ 
sible, however, from the data, gathered in this investigation, to 
prove these characters constant for either of the two so-called 
species, or to establish definite limits of variation for them. 
The limits obtained for both forms in the features under con¬ 
sideration overlap each other, which leaves, of course, no spe- 
