OSTEOLOGY OF THE HYOPOTAMIDJE. 
55 
process is very smooth, without the large prominence we see in pigs, where it enters 
deeply into the external wall of the astragalus. The inferior surface of the processus 
anterior is occupied by one tolerably broad facet for the cuboid (Plate XXXV. tig. 4, cb). 
On the posterior surface of the thickened back end is seen a groove for the tendon of 
the musculus plantaris, which, as in all Ungulata, was certainly developed as a flexor 
digitorum ; this groove is shallower than in the Suina. 
Dimensions of Calcaneum. 
Hyopotamus, 
from Puy. 
Hiplopus, 
from Hordwell 
(Plate XXXV. 
fig. 4). 
Length 
95 
96 
Height at the fibular facet 
37 
47 
Length of the processus anterior 
34 
40 
Greatest breadth 
22 
23i 
The astragalus (Plate XXXVII. fig. 21). — This is an extremely characteristic bone 
for determining the natural afnnitiesof Ungulata; in Hiplopus it is very nearly like as 
in Hyopotamus , and conforms to the general shape of this bone through the whole 
range of Paridigitata. It has a double pulley — a proximal one, fitting the distal ends of 
the tibia and fibula, and a distal pulley fitting the navicular and cuboid. In all Impari- 
digitata the astragalus has only one pulley, on its upper or proximal end, while the lower 
or distal is flattened, though articulating with the same two bones. (In Macrauclienia 
and in the Horse, the distal surface of the astragalus articulates only with the navicular 
bone.) This character of the astragalus is one of the best by which to distinguish at a 
glance the two series of Ungulata; and we knowof no living or fossil animal which presents 
an astragalus linking these two divisions together. True, in the astragalus of Hippopo- 
tamus (Plate XXXVII. fig. 9, a ), owing to the broadness and thickness of all the tarsal 
bones, the inferior pulley is so exceedingly low that it reminds one of the same distal 
surface of the astragalus in Bliinoceros (see De Blainville’s plates) ; but this likeness 
seems to be only an analogical one, both animals having heavy tarsal bones adapted 
very nearly to the same function. 
Although extremely like in general shape to the astragalus of a pig, that of the 
Hyopotamus is proportionately much higher, the distal and proximal pulleys being divided 
by a larger interval than in the Suidae. That part of the distal pulley which fits the 
cuboid is perhaps produced a little more downwards than in the Suina, though the 
difference is very slight. In comparing the large astragali of the didactyle Hiplopus 
with the smaller ones of Hyopotamus , we find very few differences indeed ; but it may 
be mentioned that the proximal pulley of the two-toed form is relatively a little higher 
and enters more deeply into the distal end of the tibia (Plate XXXVI. fig. 7), which may 
be due to the greater narrowness of the reduced foot, requiring a more close articulation 
with the tibia. 
