54 
DE. W. KOVALEVSKY ON THE 
of the astragalus, had got to the outside of it, and were articulated with the fibula. Such 
a view of this homology is still more strengthened if we consider the relation between 
the two bones in Anoplotherium (Plate XXXVII. fig. 11, and De Blainville, Anopl. iv.) ; 
in this animal the fibular facet of the calcaneum is still covered by a process of the astra- 
galus; it seems as if it were just one of the stages of the progressive march of the 
fibular process from under the outer pulley to the outer side of the astragalus. 
As all Imp ari d iff da ta have two chief astragalean facets on their calcaneum (Plate 
XXXV. fig. 4', e and i), and no facet for the fibula, and as all Paridigitata having a 
fibular facet on their calcaneum (fig. 4 , ff) have only one chief astragalean facet, it 
seems natural to say that the second (external) astragalean facet of the Imparidigitate 
calcaneum is not lost in the Paridigitata, but has been made use of for the formation of 
the fibular facet. But there seems to exist an objection to this view ; and it is furnished 
by the Macrauchenia , the strange South-American form, which seems really to be a sort 
of Imparidigitate Camel. The calcaneum of Macrauchenia , however, is not known ; but 
the astragalus figured by Professor Owen in the ‘ Zoology of the V oyage of the Beagle ’ 
(plate xiv. fig. 4) clearly shows that there were two principal facets for the calcaneum, 
as in all typical Imparidigitata. Now the fibula has also on its distal extremity an 
articular facet for the calcaneum* ; and this facet presupposes unerringly a similar fibular 
process on the calcaneum, as it exists in the Paridigitata. If this should be really the 
case, then the existence of two astragalean facets and a third fibular one will perhaps 
invalidate the view as to the homology explained above, and it would in this case stand 
thus: — that the single large astragalean facet in the calcaneum of the Paridigitata is 
homologous to both facets of the Imparidigitata (e+b fig- 4'), the fibular facet of the 
former being a new superadded character, not found in any of our living or fossil Impa- 
ridigitata, but exhibited by the South-American Macrauchenia. So that, even with the 
large material in our hands we cannot quite certainly determine the homology of different 
parts of such an important bone as the calcaneum in both the chief series of Ungulata ; 
and as we have no forms linking these two typically different calcanea together, this is 
one proof more of the very ancient separation of this order into its two principal groups. 
The calcaneum of Hyopotamus (figured in \ nat. size, Plate XXXVIII. fig. 21) and of 
Dipjlopus Aymardi (Plate XXXV. fig. 4, nat. size) resembles very much that of a pig, 
with some slight differences. The sulcus which divides the fibular facet (ff) from the 
astragalean is very deep ; this last facet has a slight angular rising or ridge nearer to 
its inner border, which fits into a corresponding slight depression in the posterior 
(calcaneal) surface of the astragalus; besides, as in all Ungulata, there is, at the inferior 
and inner border of the processus anterior, a smooth and large surface (Plate XXXV. 
fig. 4, a s ) by means of which the inner wall of the calcaneum fits to the outer border of 
the inferior pulley of the astragalus. The fibular process (ff) is not so high, but much 
deeper an tero -posteriorly than in the Suina ; and we have seen that the distal extremity 
of the fibula is also extended in the same direction. The inner surface of the fibular 
* Voyage of the Beagle, p. 51. 
