Lehmann—Variations in Form and Size i of Cyclops. 289 
In all the specimens of C. brevispinosus, the one constant 
quantity among the characters under consideration is the arma¬ 
ture of the fourth swimming foot, the terminal joint of the 
inner branch bearing always upon its external side a distinct 
spine, whereas C. americanus hears: frequently at this point a 
seta. The furca of brevispinosus varies greatly in length, the 
limits of the specimens examined being .096 mm., and .175 
mm., the greatest number of any one length being at .15 mm., 
whereas an average length would be .135 mm. The width of 
the furca at its base varies from .037 mm. to .054 mm., the 
greatest number of any one width being at .045 mm., which is 
about an average width. The width of the terminal end of the 
furca varies from .02* mm. to .035 mm., the greatest number 
of any one width being at .025 mm., which is also nearly an 
average width. These figures show a greater variation in 
length than in width. The outer terminal spine of the furca, 
claimed by Herrick to be a distinctive feature, shows great 
variation, first in length, from .038 mm. to .07 mm., the great¬ 
est number measuring .065 mm., whereas an average length 
would be .054 mm.; and second, it varies in form from a thick, 
very blunt spine to a slender, seta-like spine, the 1 greatest num¬ 
ber being thick at the base but tapering rapidly toward the 
apex. The two extreme forms are least in number, the thick, 
blunt spine being least frequent of all the forms. Hone of the 
specimens examined bear a distinct seta at this point, such as is 
occasionally found on americanus; and no specimens of amer¬ 
icanus examined bear a thick, blunt spine such as is sometimes 
found on brevispinosus. However, these two forms are ex¬ 
tremes and may easily have been developed by peculiar' condi¬ 
tions from a common form, since the slender tapering spine 
common, to both species is found most frequently in both. 
Forbes, ’97, has noted similar variations in this spine; he says: 
“I have seen considerable variations in this spine and have ob¬ 
served all the gradations between it and the usual slender spine 
of C. insectus. I think, however, that. C. brevispinosus should 
be considered a good variety.” 
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