OSTEOLOGY OF THE HYOPOTAMIM. 
53 
the other representatives of the same order. Fortunately the bones of the tarsus are 
much more numerous than the carpal bones both at Puy and in England ; and I have 
been enabled to restore it completely, with the exception of the first cuneiform, which 
is wanting. 
Calcaneum (Plate XXXY. fig. 4, Plate XXXVII. fig. 21, c). — This bone presents a 
remarkably uniform shape through the whole range of Paridigitata ; in Hyopotcimus 
(Plate XXXVII. fig. 21, c ) and Biplopus (Plate XXXV. fig. 4) it is very similar to the 
calcaneum of a pig. Owing to this uniformity of the calcaneum in the Paridigitata, we 
must extend our comparison to the other series, or Imparidigitata, if we intend to find 
out what are really the characteristic and essential features of a Paridigitate calcaneum. 
The general shape of the calcaneum in the entire order of Ungulata is, on a cursory 
glance, very similar ; indeed, to find the typical differences we must enter into a deeper 
analysis of the bone in question. If we put before us some calcanea belonging to 
animals of the Imparidigitate series, for instance that of a Palceotherium, Tapir, Rhino- 
ceros, or Horse, and try to compare them with those figured in our plates, or, still 
better, with the calcaneum of a Pig in natura , we shall immediately see that in the 
first* the upper and outer edge of the calcaneum, where it bends down to enlarge into 
the sustentaculum, forms a large articular projection (fig. 4', e), which enters below 
and behind the external pulley of the astralagus ; to the inner side of this articular 
facet we have the sulcus sustentaculi (fig. 4', s), and, on the other side of this sulcus, 
another large articular oblong internal facet for the astragalus (fig. 4', i). In corre- 
spondence with this, the posterior part of the astragalus of all Imparidigitata presents 
two large principal facets for the calcaneum — an outer facet, under the outer pulley, 
and an inner oblong facet. 
Looking now, with a view to a strict comparison, at the same upper edge of the cal- 
caneum of a Paridigitate (Plate XXXV. fig. 4 ; Plate XXXVII. fig. 21), we see that, 
at the point where the anterior edge bends down into the sustentaculum, it is divided 
into two parts by the sulcus sustentaculi ; the external part, being the direct prolonga- 
tion of the anterior edge, forms a prominent articular facet for the fibula (Plate XXXV. 
fig. 4, ff) ; the inner part, being situated on the sustentaculum proper, gives a large 
single facet for the posterior part of the astragalus (as). Considering these two facets 
in reference to the sulcus sustentaculi, I thought that the outer astragalean facet of the 
calcaneum of the Imparidigitata (e, fig. 4') ought to be homologized with the fibular 
facet of the calcaneum of the Paridigitata (fig. 4, ff), and the inner oblong astragalean 
facet of the calcaneum of Imparidigitata (i, fig. 4') with the single astragalean facet on 
the sustentaculum of Paridigitata. Comparing in natura the calcaneum of a pig and 
a horse, this is the view which seems most natural : it looks quite as if the outer astra- 
galean facet of the Horse’s calcaneum, instead of being buried under the outer pulley 
* To make my comparisons better understood I figure tbe front view of an Imparidigitate calcaneum (Plate 
XXXY. fig. 4') ; it belongs to Anchitherium. Tbe calcaneum (Plate XXXY. fig. 4') is a left one, while tbe two 
calcanea (Plate XXXY. fig. 4 and Plate XXXYII. fig. 21) are right. 
i 2 
