424 The American Naturalist. [May, 
of the tongue and groove articulation between the metapodials 
and first phalanges. In alighting on a didactyle foot, the toes 
are naturally spread, the result being to throw both the first 
phalanges away from the median line, or axis of impact, in 
divergent directions. The result of this impact is to produce 
upon each metapodial condyle as in the case of the humerus, 
an external roller of smaller diameter than the rest of the 
condyle, and separated from it by an abrupt crest. In both 
humerus and metapodial bones these crests are accentuated by 
a pinching process during flexion and extension. This is 
produced by the longitudinal torsion which results in all 
limbs in motion from the arrest of the outward rotation of the 
foot by the earth on alighting. The pinching of a keel by 
its groove takes place at all points in the length of the former, 
reached by the opposite sides of the extremities of the latter 
during flexion and extension. This keel becomes acute and 
prominent in the Bodidea and extends to the anterior face of 
the condyle. This development has been apparently 
especially due to the presence of two sesamoid bones, embed- 
ded in the flexor tendons, one on each side of the middle line 
of the posterior side of the metapodial condyle. The fissure 
between these two bones has permitted the moulding of the | 
surface into a keel to fit it. That this has been the case is 
further indicated by the fact that a median trochlear surface 
exists at the distal extremity of the first phalanx in all mam- 
mals. But asingle flexor tendon crosses this articulation, and 
it contains but one sesamoid bone, to which the trochlear sur- 
face is moulded in a concave surface, as is the case of the 
patella and the rotular groove of the femur. ” 
In support of Cope’s proposition, that the development of 
crests or keels upon the plantar or palmar aspect of the distal 
end of the metapodials, is due primarily to the presence of 
two sesamoids imbedded in the flexor tendons, I can add 
that in every case where these sesamoids are present, and 
pressure is exerted by the flexure of the phalanges upon the 
metapodials, the keels are developed, and conversely, that 
where the sesamoids are absent or are present without pres- 
sure, the keels are absent. The human foot and hand are 
