194 COPE. [ VoL. III. 
with the bones of the second set (phalanges). These keels first 
appear on the posterior side of the end of the first set of bones, 
projecting from between two tendons. These tendons, in most 
mammals, contain two small bones, one on each side, which act 
like the knee-pan and resemble it in miniature, the sesamoid 
bones. These tendons and bones exercise a constant pressure on 
each side of middle line, when the animal is running or walking, 
when extension of the phalanges is most 
pronounced ; and this pressure, together 
with the concussion with the ground and the 
torsion of the limb, appears to have permit- 
ted the protrusion of the middle line in the 
form of a keel, while the lateral parts have 
- been supported and 
even compressed. 
The reptilian ances- 
) tors of the mam- 
mals do not possess 
these keels. 
Figure 36.— Part of anterior foot of Procamelus occidentalis, from New Mexico; 
from Report of Capt. G. M. Wheeler, Vol. IV., Pt. IT. 
Figure 37.— Metacarpals of Cosoryx furcatus, from Nebraska, two-thirds nat. 
size: a, anterior face; 4, posterior; c, proximal end; d, distal end. 
Figure 38. — Left fore foot with part of radius of Poébrotherium vilsoni Leidy, 
from Colorado, three-fifths nat. size; from Hayden’s Report, III., (unpublished). 
