492 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
The hind limbs, which are still functional in the oldest sea-cows 
from the lower Middle Eocene (fig. 23, I), in those of the upper 
Middle Eocene, have already become functionless (fig. 23, II). The 
pelvis, which originally consisted of ischium, ileum, and pubis, has 
degenerated. The obturator foramen has disappeared. (Fig. 23, I1.) 
The acetabulum has become smaller and rudimentary. (Fig. 23, ITT.) 
By making use of later forms we can trace step by step the course 
through which the degeneration of the pelvis and of the femur has 
Text-fig. 66. 
fqa. 
Fic. 22.—Skull of a sea-cow, Protosiren fraasi Abel, from the lower Middle Eocene of 
Egypt. Length of skull 36.5 em. (14.1 in.). <A. Skull from below. D. From above. 
C. From behind. B. Right tusk, or anterior incisor. 2 natural size. After C. W. 
Andrews. 
progressed. The pubis gradually became smaller (fig. 23, II, IIT, 
IV) and disappeared entirely in a sea-cow from the Miocene of Aus- 
tria. Finally, only a long rod of bone remains, the upper part of 
which consists of the ileum and the lower part of the ischium, as in 
the extinct Arctic sea-cow and the existing dugong. 
The South American manatee has retained a rudiment of the 
femur, but this is only about 18.5 mm. long and about 2.5 mm. thick, 
while the animal itself reaches a length of 3 meters (10 feet). (Fig. 
24.) 
