284 Wisconsin Academy of Sciences , Arts , and Letters. 
Table I shows a record of measurements of thirty-five speci¬ 
mens of brevispinosus. The date and place of collection was 
indicated when possible, and a record was taken (1) of the 
length of the furca along an axis ah, as represented in the ac¬ 
companying diagram; (2) the width of the furca along the line 
cd; (3) the width of the furca along the 
Yline xy; (4) the form and length of the 
outer apical spine of the furca; and (5) the 
armature of the terminal joint of the inner 
branch of the fourth swimming foot. Table 
II shows a record of the same features in 
eleven specimens of C. americanus having a 
constant armature of the fourth swimming 
feet as described by Professor Marsh. Plates 
XXXII and XXXIII show drawings of the 
furca and its outer apical spine, and of the 
terminal joint of the inner branch of the fourth swim¬ 
ming foot of eight specimens supposed to be C. americanus , 
but not answering to Marsh’s description. Diagrams 1, 2, 
3, and 4 of Plates: XXX and XXXI show the curves 
of variation for the furca and the outer apical spine of 
the furca of C. brevispinosus. It was impossible to rep¬ 
resent in this; manner the variations: in C. americanus on ac¬ 
count of the extensive variation in the small number of speci¬ 
mens attainable. Plate XXXIII, Fig. 5, shows a fourth 
swimming foot of C. brevispinosus , and Fig. 6, a fourth foot 
of C. americanus of the type described by Marsh, ’92. Fig. 
1 of Plate XXXIII shows an extreme form of furca in G. 
brevispinosus, and Fig. 8 shows an extreme form for C. ameri¬ 
canus. 
