OSTEOLOGY OF THE HYOPOTAMIM. 
77 
the outer side of the unciform ; and this lateral position is more pronounced in Dicoiyles 
than in the Hog, in consequence of the greater reduction of the former. 
Moreover, if we examine attentively the proximal surfaces of the two middle meta- 
carpals in Dicotyles, we shall see that they are much more joined together than in the 
common Hog or any other Sus ; their inner flat sides are so closely united that it will 
require only a little step further to make them coalesce ; and as the os magnum and 
trapezoid are now both resting on one metacarpal they cannot remain long distinct, 
but must coalesce : if this should occur, we should have a structure nearly analogous 
to, and hardly to be distinguished from, that of a typical ruminant. 
If we turn now to the structure of the ])es in the Paridigitata with tuberculated teeth, 
or Suina, we shall meet with precisely the same phenomena ; and the homologous bones 
of the manus and pes undergo a strikingly similar course of variation. 
The two middle digits of th e pes interlock, in the usual way, by a process from the 
fourth metatarsal, which fits into an excavation of the third. The cuboid supports the 
two outer digits, the fifth and fourth ; the third digit is supported by the third cunei- 
form. If we compare the third metatarsal of the recent Hog (Plate XXXVII. 
fig. 12, hi.) with the same metatarsal of Palceoclicerus, fig. 12', and of Cheer omorus , we 
perceive just the same difference as in the homologous bones of the manus. While in 
Palceoclicerus the third metatarsal is confined entirely to the third cuneiform*, leaving the 
second cuneiform for the support of the second metatarsal, in the Hog (fig. 12, hi. c 2 , c 2 ) 
this third metatarsal is greatly enlarged ; it has pushed the second digit away, 
and encroached on nearly the whole of the distal surface of the second cuneiform, 
leaving only a very narrow facet of this bone for the second metatarsal (fig. 12, ii. c 2 ), 
which is now chiefly supported from behind by the wedge-shaped first cuneiform. 
On the outer side of the pes, the enlargement of the fourth metatarsal has taken the 
greater part of the distal surface of the cuboid, the fifth metatarsal being pushed very 
much backwards, and being supported partly by a small facet on the distal surface of the 
cuboid, and partly by a posterior prolongation to the beak-like downward process of the 
same bone. The pes of the Dicotyles shows us the same disposition in an exaggerated 
form; the inner side of the third metatarsal (Plate XXXVII. fig. 13, hi.) has completely 
occupied the second cuneiform, so that the second digit is supported only from behind by 
the first cuneiform. On the outer side, the fifth digit is completely lost ; or, if a rudiment 
of the fifth metatarsal remains, it is generally a flat elongated bone attached to the outer 
side of the fourth metatarsal, and having no articular surface on the cuboid, which is 
taken up by the over-developed fourth metatarsal. Besides, the two middle digits 
have coalesced in the whole upper half of their length, simulating the cannonbone 
of a Buminant. The navicular and cuboid are still separated, but are very firmly 
pressed together; and now, both these bones, having the coalesced metatarsal below 
them, cannot have much separate movement, and their coalescence, as well as that of the 
* As the hind limbs are always more reduced than the fore, it seems that, even in Palceoclicerus, the third 
digit encroached in a perceptible way on the second cuneiform. 
M 2 
