70 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
the crown is marked by a broad shallow concavity opposite the second 
external groove. The crowns of the lower molars and molariform pre- 
molars curve outward, and, as in the superior series,, are invested with 
a thin layer of cement. 
The milk dentition is unknown. 
Skull (PI. I, figs. 1-4, 6; text fig. 13). — The skulls in the collection at 
Princeton University and the American Museum of Natural PI istory belong 
to individuals of approximately the same age and show but a small amount 
of variation in size, so that the measurements given in the description of 
H. mirabile may be regarded as a fair average. 
In side view (PI. I, fig. 1) the upper profile of the skull is broadly con- 
vex, the surface sloping gradually forward and rather rapidly backward 
from the parietal eminences. The orbit is approximately central and 
almost entirely inclosed posteriorly by the strong postorbital frontal process 
and the abruptly truncated anterior extremity of the zygomatic process of 
the squamosal. Unlike Protypotheriwn and Inter atherium, the maxillary 
is entirely excluded from taking any part in the orbital border by the 
malar, the free border of which is produced anteriorly as a broad plate 
overhanging the preorbital expanse of the maxillary (text fig. 13), bound- 
ing externally a deep fossa and, in side view, almost concealing the infra- 
orbital foramen, which lies above the anterior margin of the first molar. 
Anterior to the lachrymal, a small portion of the maxillary enters into the 
lateral surface of this plate. The lachrymal is about equally divided into 
orbital and extraorbital moieties, with the tear duct double, one canal open- 
ing within the orbit behind the rather large lachrymal tubercle and the other 
on the orbital margin below the tubercle. The maxillary is in contact 
with the frontal by a small process extending between the nasal and lach- 
rymal (PI. I, fig. 2). 0 This process has less posterior extension than in 
either Protypotherium or Inter atherium, in which it is produced well beyond 
the anterior margin of the orbit, while in Hegetotherium it does not extend 
much beyond the fronto-nasal suture. A small process of the frontal is 
received between it and the nasal. Anteriorly, the nasal and maxillary are 
in broad contact, preventing the premaxillary from touching the frontal. 
Posteriorly, the maxillary gives rise to a long narrow process applied to 
the inferior inner surface of the zygoma and terminating a short distance 
anterior to the glenoid cavity. The posterior border of this zygomatic 
process of the maxilla originates opposite the posterior margin of M-. In 
