638 THE AMERICAN NATURALIST. 
seen in the dried skeleton. The two sacral ribs on the left 
side are about equal in size and share alike in the support of 
the ilium. | 
The other four specimens having three sacral ribs, listed in 
Table II, each have paired sacral ribs on the nineteenth verte- 
bra, while the unpaired sacral rib is borne on the right side of 
the twentieth vertebra in two of them, and. 
on the left side in the other two. Twenty- 
five specimens are listed which have an 
asymmetrical sacrum with but two sacral 
ribs, and in nine of these the sacral rib of 
the right side is further cephalad, while in 
the remaining sixteen it is the left sacral rib 
which is in advance. It seems curious that 
in the first ten specimens which were reported as having this 
condition of the sacrum, nine should have had the left side in 
advance, while in the next fifteen over half of them should have 
the right side in advance ; and again it is singular that, of six 
specimens in which the eighteenth vertebra is invaded, five of 
them should have the right side in advance, while in the 
remaining nineteen cases fifteen should have the left side in 
advance. It is evident that data from a much larger number 
of specimens must be obtained before trustworthy generaliza- 
tion can be made. 
One individual found during the past winter and listed above 
has a symmetrical sacrum on the twenty-first vertebra, ae 
dition not previously recorded, I believe; and in one specimen 
described by Parker this vertebra was invaded on the right 
side; but these two specimens are the only ones, SO far as I 
know, in which the twenty-first vertebra is invaded by the 
sacrum. 
I am under obligations to Mr. Ralph G. Mills, a student at 
the University, for making the drawing for the figure, and also 
for the examination of a considerable number of the specimens 
studied. 
UNIVERSITY OF ILLINOIS. 
Ld 
