OSTEOLOGY OF THE HYOPOTAMIDiE. 
67 
reduced skeleton, we still possess nearly all the intermediate stages between four com- 
pletely developed and distinct metacarpals and metatarsals, and their reduction to two 
coalesced middle ones. Besides, the links which are wanting in the fossil state are often 
furnished by the living forms in the course of their individual development before they 
reach the adult age. Such links are presented by the Tragulidae and the posterior extre- 
mities of Dicotyles and IlyomoscJius. If we examine the bones of their feet at an early age, 
when the fourth and third digits are yet distinct, we shall see exactly the same relation 
as is laid down in our general rule. But even after the complete coalescence of these 
two metacarpals and metatarsals, some traces of the original disposition are to be detected. 
Examining the fore “ cannon” of the typical Buminantia (especially Deer) we shall see 
that one half of the proximal surface, answering to the third metacarpal, is a little higher 
than the other half, and always presents a produced ulnar margin, by means of which 
it abuts against the unciform ; the same is to a certain extent seen in the back cannon, 
where the proximal surface for the cuboid is often a little lower than the surface for 
the third cuneiform. 
These two middle digits (third and fourth) form the chief basis of the foot of a Pari- 
digitate Ungulate ; to these are added, on the inner and outer sides, one digit more, the 
second and the fifth. But so much is the skeleton reduced in the existing Paridigitata, 
that there exists at present only a single form (the Hippopotamus), represented by a 
single species (or two, as Choeropotamus liberiensis appears to be distinct), in which these 
two lateral digits have a true functional importance ; and although the lateral digits still 
exist in all the Suidse, Tragulidae, and Hyomoschus , they have no real functional import- 
ance. Even in the Suidse the two middle digits are so largely developed in comparison 
with the lateral ones, and all the bones of the carpus and tarsus have been so completely 
taken for the use of these two middle digits, that we shall be guilty of no exaggeration 
in stating that the Suidse might lose the lateral digits without their locomotion being at 
all impaired. We even witness this process going on in Dicotyles — in which the lateral 
digits are still more reduced than in the true Suina, and they begin to disappear altogether, 
beginning at the external metatarsal. If the lateral digits are still retained in the Suidse, 
it is chiefly owing to the fact that the Hogs live generally in marshy places and on 
muddy river-banks, where a broad foot is of great importance for not allowing them to 
sink deeply into the mud. But if, by some geological change, their habitat should be 
transformed into dry grassy plains, there can be no reasonable doubt that they would as 
readily lose their lateral digits as the Palseotheroids have lost theirs (perhaps by an 
analogous change of habitat) in becoming transformed into the monodactyle Horse. 
Should this occur only in a limited locality, and under circumstances admitting of no 
migration (for instance, on a large island), it is very possible that the Suidse of this 
particular island might lose their lateral toes, while others, which continued to li\e 
under the old conditions, would retain theirs. If this should really occur, we should 
have two groups of the same family, quite similar as to the large bones of the 
skeleton, but dissimilar as to the structure of their feet, which is just what we witness 
MDCCCLXXIII. L 
