OSTEOLOGY OF THE HYOPOTAMHLE. 
85 
projections of the two middle digits that must in reality articulate together are sepa- 
rated in the drawing.) The inner or tibial side of the third metatarsal shows an oval 
facet for the articulation of the second digit ; a like facet is seen in the Suina and 
Hippopotamus, only much shorter. 
The fourth metatarsal (Plate XXXVIII. tig. 1, iv.). — What at once strikes the 
observer on looking at this metatarsal, is the large projection of its tibial side, which 
enters into the corresponding pit of the third metatarsal ; a little below it the inner 
surface of the bone is very rugose and bulging a little inwards. The proximal surface 
(fig. 2', iv.) is slightly concave at its fore and inner part, and somewhat raised in the 
postero-external angle (the elevation is indicated in fig. 2' by a deeper tint) ; this raised 
joint fits exactly into the postero-external concavity of the distal surface of the cuboid 
(fig. 10, iv.), while the remaining, and slightly convex, cuboidal surface is fitted to the 
slight concavity of the inner and fore part of the proximal surface of the fourth meta- 
tarsal. 
The outer side of the fourth metatarsal has a lengthened oval facet for the articula- 
tion of the fifth digit ; such a facet is to be seen in the Hog, only a little shorter. 
Whether the fifth digit articulated with the fourth by a second facet I am unable to say, 
as my specimen is a little defective ; but very probably it did. 
As the two middle digits, in their general shape, bear a great likeness to each other, 
all I shall say of one will be referable to the other. Their symmetry is somewhat 
disturbed by the slight bulging of the inner side of the fourth metatarsal ; although, 
if we look at the anterior surface of the whole pes, this slight disturbance does not 
interfere with the general symmetry of the two middle digits. 
The section of the middle digits (Plate XXXVIII. below, fig. 1) has a flattened tra- 
pezoid outline, especially if we take it in the upper part, where the posterior projection 
is prolonged downwards as a flattened platform in the upper half of the posterior surface 
of the metatarsals; towards the middle this platform subsides, and we have a more 
rectangular section. This flatness of the metatarsals is very striking in comparison with 
the rounded outline of the metatarsals in the didactyle genus ; but one of the living 
Paridigitata, the Hippopotamus, has even much flatter metatarsals *, their thickness 
being only half of their transverse breadth. The outer margins of both middle meta- 
tarsals are made conspicuous and angular by the pressure of the completely developed 
lateral digits; their anterior surface is therefore very flat, even more so than in the living 
Hippopotamus. The middle metatarsals preserve a uniform breadth along their entire 
length, and we see no such conspicuous broadenings of the distal ends as in the didactyle 
form. The restored pes (Plate XXXVII. fig. 21) does not show this very clearly, as the 
complete digits from Puy were much disfigured by pressure ; but it could be readily seen 
on some well-preserved fragments of the distal extremities. The distal end is quite 
* I find in a Hippopotamus with epiphysed bones that the fourth and third metatarsal have 32 mm. trans- 
verse breadth each, and 16 and 17 mm. thickness or depth, while in Hyopotamus the breadth is 16 each and the 
depth 11 mm. 
N 2 
