82 
EE. W. KOWALEVSKY ON THE 
The general shape of both middle digits in Hyopotamus is very flat in front, with two 
tolerably sharp edges, which form the outer and inner border of each metacarpal ; the 
two inner, or contiguous, edges are formed by the metacarpals being pressed against each 
other in the axial line, while the outer edges result from the pressure of the lateral digits 
on the two middle ones from without and within (see fig. 5, and section). In the 
Hiplopus, as we shall see hereafter, we have only the contiguous edges, while the outer 
sides of both metacarpals are rounded and smooth, there being no lateral digits. The 
section of the four metacarpals given belo-v fig. 5 may give an idea of the flatness of 
these bones in the middle : it may be possible that they are somewhat flattened by 
pressure ; but in the living Hippopotamus the metacarpals are perhaps relatively as flat. 
The length of the third metacarpal (drawn in outline) from Puy is given by a com- 
plete specimen in the collection of M. Aymakd. I have also several rolled specimens 
of both middle and internal lateral digits from Hempstead. 
The distal ends of the two middle metacarpals (Plate XXXVII. fig. 20) are quite 
smooth in front ; but on the posterior or palmar surface of each a median prominent ridge 
is seen corresponding with a sulcus on the proximal extremity of the first phalanx. In the 
Hog, this ridge goes round the whole distal end of the middle metacarpals. The outer 
half of this distal end is a little shorter than the inner, though the difference is not, by 
far, so great as in Suina ; and the first phalanges being modified accordingly to fit the dis- 
tal extremity of the metacarpals, the outer half of the proximal surface of each is slightly 
higher than the inner. This corresponding inequality of the distal ends of the meta- 
podial bones and the proximal surface of the first phalanges is seen in many fossil genera, 
but is most manifest in the recent Suina. The purpose of this arrangement seems to be 
the compression of the metacarpals of the middle digits and the approximation of their 
distal ends. The two middle digits diverge in treading on the ground ; but, by the same 
action, their proximal extremities tend to converge ; and, owing to the special disposition 
of the metacarpo-phalangeal articulation, they compress the two metacarpals or meta- 
tarsals together. In such animals as the Hog, in which this disposition is strongly de- 
veloped, and aided by a special adaptation of the tarsal, and to some extent carpal, bones 
to bring together the proximal extremities of the two middle digits, these two practically 
work like the cannon of a Ruminant. In Hyopotamus, however, this disposition is 
only indicated, while it is developed much better in Diplopus. 
The lateral digits of the Hyopotamus are very well developed, and, in fact, besides the 
Hippopotamus , we know of no animal in which they are so complete and relatively large 
as in Hyopotamus. It is possible that in Anthrac 'other mm they took even a more im- 
portant part in locomotion ; but the complete limbs of Anthracotherium are not fully 
known at the present time. 
The inner or second metacarpal (Plate XXXVII. fig. 20, & Plate XXXVIII. fig. 5, ii.) 
is represented in all collections I have visited only by its proximal half. The upper or 
proximal extremity of the second digit presents an elongated and concave articular 
surface for the distal face of the trapezoid ; its ulnar margin, as seen in fig. 5, is 
blunted by an oblique facet, which must have abutted against the os magnum. 
