0. C. Marsh—Polydactyle Horses. 503 
much less likely to be true. The cases of apparent reversion 
are of especial interest, and it is important to place on record 
any information in regard to a so that they may be com- 
pared with extinct allies of the hors 
ri cases of extra digits in the bora so far as at present 
known, show that these appendages make their appearance 
more frequently on the fore feet than on the hind feet. This 
is precisely what a wert of the fossil forms of equine mammals 
would lead us to antici 
more frequent occurrence on the inside of the main eet while 
the outer splint remains rudimentary. This, it must be con- 
fessed, is directly opposed to the general law of reduction in 
the ungulate foot, which, briefly stated, is, that of the five 
original digits, the first or inner one, first ‘disa appears ; next the 
fifth, or outer one; then, the second ; and last of all the fourth. 
The third always remains, as in the horse. It would, peer. 
be naturally expected, that when only one additional digit w 
present, it would be on the outside of the fore foot. 
e tendency to interference would seem to be another reason 
against the retention of the inner digit. Possibly the addi- 
tional protection which an inside hooflet would receive, might 
more than oe this influence. Again, the above 
of greater use than the fourth, and hence was ices ps | 
an ancestor of the horse may yet be found with the second 
and third toes alone developed. 
In considering these double hoofs of the horse, and with 
them the well known cleft in the coffin bone of recent and 
extinct equines, it is important to understand that in no case 
do they indicate any approach to the true — type, 
as some authors have supposed. The difference between the 
pobictanett te or ‘odd-toed,” and artiodact ela or “‘even-toed,” 
structure is a profound one, extending to nearly every part of 
the skeleton, and marking two distinct groups lates. 
The number of toes has really nothing to do with the true dis- 
tinction, and hence the terms in use are especially gregory A 
Th ifference, so far as the feet are concerned, is, that 
the perissodactyle type the axis of the limb passes through the 
middle of the third digit (Mesawonia), while in artiodactyles it 
is outside of this digit (Paraxonia), between it and the fourth. 
If, now, we turn back to the early ancestors of the horse 
for an explanation of the or yeegiea d digits which so often 
make their appearance, we shall not look in vain, especially in 
this country. America is the original home of the horse, and 
