Sinclair: typotheria of the santa cruz beds. 87 
and the first and second molars are convex internally, while externally 
they are divided by a deep groove into two crescentic lobes. M3 differs 
from the teeth preceding in the greater antero-posterior diameter of the 
crown, which is externally trilobate and broadly concave postero-internally. 
A layer of cement is present as in the superior series. 
The milk dentition is unknown. 
Skull (PL X, figs. 1-4; text fig. 1, C). — The skull is most rabbit-like 
in appearance, due to the convexity of the dorsal profile, the slender ros- 
trum, long diastema, large prominent orbits, slender zygomatic arches and 
great posterior depth of the mandible. Dorsally, the upper profile slopes 
forward sharply from the region of the mastoid dilatation, where it meets 
the plane of the occiput at an acute angle. The slender rostrum is 
grooved longitudinally from the region of the infraorbital foramen to the 
margin of the premaxillary above the alveolus of the median incisor. The 
premaxilla is long and heavy, without ascending process. Laterally, it 
is broadly grooved and perforated by numerous foramina. Dorsally, it is 
in contact with the nasal, but not firmly united therewith. Inferior ly, 
the long incisive foramina extend beyond the premaxillo-maxillary suture, 
unlike Hegetotherium ( cf. Pis. I, fig. 3 ; X, fig. 3). The facial portion of 
the maxillary is also excavated longitudinally. Its thin walls are perfor- 
ated by a net-work of foramina, recalling this region in the rabbit. The 
maxillary is excluded from the orbital margin by the lachrymal and malar, 
as in Hegetotherium. It forms the larger part of the orbital floor and 
appears as a narrow zone in front of the lachrymal at the margin of the 
plate-like elevation formed from the maxillary, lachrymal and malar, which 
bounds the orbit anteriorly. A thin vertical lamina developed from the 
maxillary incloses a groove, at the lower extremity of which the infra- 
orbital foramen lies. The zygomatic process of the maxillary extends 
almost to the glenoid cavity, its posterior border originating some distance 
back of the last molar. The facial expansion of the lachrymal is large, 
but, owing to the number of foramina perforating it, this portion of the bone 
is usually more or less broken. A prominent tubercle is developed on its 
orbital margin, concealing in external view the lachrymal duct. Contact 
between frontal and lachrymal is reduced by a narrow bar of the maxillary. 
The orbits are almost circular, very large and prominent and but slightly 
constricted posteriorly by the slender styliform postorbital processes. The 
zygomatic arches are slight, resembling those of Hegetothevium in the 
