1893,] Evolution of Metapodial Keels of Diplarthra. 425 
excellent examples of this. In the foot there are two sesa- 
moids developed in the tendons of the flexor brevis hallucis as 
they pass over the end of the metatarsal of the great toe, to be 
attached to the phalanx. Now in the act of walking the 
greater part of the weight of the body falls upon this digit and 
as the heel is raised and the footis brought into a more or less 
vertical position, these tendons are put upon the stretch and 
pressure results; in this metapodial, therefore, wé find the 
keel present. In the other digits the sesamoids are absent 
and there is no keel developed. In the upper extremity there 
are sesamoids developed in the tendons of the flexor brevis pol- 
licis at the metacarpo- phalangeal articulation, but owing to 
lack of pressure the keel is absent. Again in the Spider 
Monkey there are two sesamoids present in the tendons of the 
short flexors of the great toe and the keel is developed, while 
in the other digits there are no sesamoids and no keels. In 
other species of monkeys on the other hand such as the Mac- 
aques, the sesamoids are present and the keels are developed 
upon all the metapodials. These monkeys are saitl to be less 
arboreal in their habits, which would explain the difference in 
the matter of sesamoids and keels. Among the Marsupials 
there is no patella in the Vulpine Phalanger, and here we find 
that the rotular groove of the femur is very little developed. 
= Hmilut is without the sesamoids and there is no keel, 
he fact that there are both sesamoids and keels 
in the other metapdids These cases could be multiplied 
indefinitely showing the same results. I havein fact failed to 
find a single example of a metapodial, in which sesamoids are 
present and pressure is exerted, which does not show the presence 
of the primary keel. 
I will now consider Mr. Carey’s other proposition viz: that 
the crests or keels in certain highly specialized forms reach 
around to the anterior face of the bone, and are not explain- 
able by, nor cannot be assigned to any mechanical reason that 
is obvious. I must say that when I met with this problem 
several years ago, I was somewhat at a loss to understand how 
any mechanical explanation could possibly be applied to its 
Professor Cope’s explanation of the extension of these keels in 
= 
