53^ THE OSTEOLOGY OF HY^NODON. 
oi.t strongly from the sides of the skull, as well as arching decidedly 
Ixdng iioUhe least tendency toward the assumption of the peculiar conditions foil 
i„ Ihuenodm and, to a much less marked degree, in Plerodon. The lachrymal is 
not cNpanded on the face {fide Wortnian) and the mastoid processes are very largely 
develoiHHl. In the Hymiodontidce, on the contrary, the face is not shortened, but 
rather elongated and tapering ; the zygomatic arches are straight, slender and placed 
very low down on the si.les of the skull, while anteriorly they are contmued by the 
alveolar portion of the maxillaries. This unique arrangement has attained only an 
incipient stage in Pterodon. The lachrymal forms a large facial plate m front of 
the orbit and the mastoid process is rudimentary. 
The extremities display divergences of at least equal significance. In the more 
ancient family the scapula is large, with broad, flattened acromion and largely de- 
veloped metacromion. The humerus is short and massive, with enormous deltoid 
and supinator ridges. The ulna is very massive and has a remarkably high ole- 
cranon. In Oxycena the carpal bones are of moderate width and of the form usual 
among the Carnivora, while in Patriofelis they have more the width found in Hyee- 
nodon ; me. 11 has lost its connection with the magnum. In Hyesnodon the carpus, 
especially' the cuneiform, has greatly widened and me. II has an extensive connec- 
tion with the magnum, while that of me. Ill with the unciform is much I’educed. 
Even more important are the dillerences in the pes. 
In the Oxytenidee the astragalus is depressed, flattened and hardly at all 
gr(K>ved ; it has a stout neck and the head articulates extensively with the cuboid, 
the proximal end of which thus receives a highly characteristic shape. The calca- 
neum has a short tuber and does not articulate with the fibul.a while the metatar- 
sals have a strongly divergent position, almost like the sticks of a fan. In the 
I lyernodotUidce the pes has become entirely carnivorous in type ; the astragalus does 
not articulate with the cuboid, but the calcaneum has developed a very large and 
prominent facet for the fibula, such as occurs in no other unguiculate. The meta- 
tarsals do not diverge strongly from one another, but pursue an approximately paral- 
lel course. 
1 lie group to which both the Oxycenidee and Hyeenodontidez may be traced 
back is undoubtedly the family Proviverridce . The skull-structure of such genera as 
Cy fiohycenodon and Smopa {Stypolophus) is of a kind from which that characteristic 
of tlie Oxyuemda, on the one hand, and of the Hyeenodontidee , on the other, might 
readily be derived; even the remarkable character of the posterior narcs found 
in the latter is perhaps in an incipient stage. The dentition also is of the type 
which we should expect to find in the ancestral form; this is particularly seen in 
the reduced size and transverse direction of ”1'^, the very close approximation of the 
par.i- and metacones on the upper molars and the development of a trenchant pos- 
terior riflge behind the latter. This type of molar-structure is already indicated in 
Puerco tunes and if this feature alone be regarded, the genera Deltatherium, Sinopa, 
Oxyana, Plerodon, Hycenodoii, form a complete series of transitions from the tri- 
tubercular to the exclusively sectorial pattern of crown. Various considerations, 
