1881.] on the Feet of Mammaha. 545 
(figures 2-3) originated from the Amdélypoda, or primitive Ungu- 
lata (figure 1), which first assumed terrestrial habits, while the 
Artiodactyla (figures 4 and 9-11), originating from the same 
order, long continued as mud dwellers; as witness the hippo- 
potami and hogs of to-day. The mechanical effect of walking 
in the mud is to spread the toes equally on opposite sides of 
the middle line. This would encourage the equal development of 
the digits on each side of the middle line, as in the cloven-footed 
types. In progression on hard ground, the longest toe (the 
third) will receive the greatest amount of shock from contact 
with the earth. There is every reason to believe that shocks, if 
not excessive, encourage growth in the direction of the force 
applied. This is strongly suggested by the relations between the 
length of the legs and the rate of speed of animals; and the 
lengths of the teeth and their long-continued use. Certain it is 
that the lengths of the bones of the feet of the Ungulate orders 
have a direct relation to the dryness of the ground they inhabit, 
and the possibility of speed which their habitat permits them, or 
necessarily imposes on them. 
II. The hinge between the first and second series of tarsal 
bones in the Artiodactyla, may be accounted for by reference to the 
habits which are supposed to have caused the cloven-footed char- 
acter. Observation on an animal of this order walking in mud, 
Shows that there is a great strain anteroposteriorly transverse to 
the long axis of the foot, which would readily cause a gradual 
loosening of an articulation like that connecting the two series 
of tarsals in the extinct Amblypoda. Any one who has examined 
this part of Coryphodon will see that a little additional mobility 
at this point would soon resemble the second tarsal joint of the 
hogs. In the case of animals which progress on hard ground, no 
Such cross-strain would be experienced, and the effect would be 
to consolidate by flattening the fixed articulation. 
IIT. The trochlez. These prominences, which form the tongues 
of the tongue and groove articulations, exhibit various degrees of 
development in the different Mammalia. Those of different parts 
of the skeleton coincide in their condition in any one type of am- 
bulatory Mammalia, and so may be all considered together. This 
fact suggests strongly that they are all due to a common cause. 
They are all imperfect in the Rodentia and Carnivora (figures 
5-6) (except the Leporide, which are especially characterized by 
VOL. XV.—No, vil, 38 
