SINCLAIR : TYPOTHERIA OF THE SANTA CRUZ BEDS. 
7 1 
Protypotherium it arises farther forward opposite the posterior border of 
M-, while in Pachyrukhos its origin lies a considerable distance posterior 
to the last molar. The premaxillse are heavy, increasing in width superiorly, 
with the ascending process proportionately shorter than in Protypotherium. 
On the palatal surface the premaxillae almost entirely inclose the incisive 
foramina. The arches are heavy, their lateral surface being formed entirely 
by the squamosal and malar in contrast with the structure in Protypo- 
therium and Interatherium , in which the maxillary enters largely into the 
lateral surface of the arch. In Hegetotherium the maxillary enters into 
the inferior inner surface of the arch, sending a narrow process applied to 
the malar almost as far posteriorly as the glenoid 
cavity. The deep, narrow zygomatic process of the 
squamosal terminates abruptly at the orbital rim 
and, with the postorbital process of the frontal, al- 
most closes the orbit posteriorly. The malar ter- 
minates a short distance anterior to the glenoid 
cavity, while in Protypotherium it bounds the glen- 
oid cavity laterally. The latter presents posteriorly 
as well as inferiorly. It is much wider than long, 
concave in section from side to side and convex 
antero-posteriorly. The postglenoid process, which 
is closely fused with the spout-like, posteriorly di- 
rected auditory meatus, is separated from the glen- 
oid cavity by a deep fossa for accommodation of 
the non-articular portion of the mandibular condyle 
when the mouth is widely opened. The mastoid is 
dilated, inclosing a large oval chamber (PI. I, fig. 
6) connected by a canal with the tympanic cavity. 
The sutures between the mastoid and the squamosal 
have been completely obliterated by fusion. This 
is usually also the case with the suture between the 
squamosal and the auditory meatus, but in No. 9223 American Museum 
(text figs. 14, 16) the latter is clearly apparent, as is also the mastoideo- 
posttympanic suture. 
Superiorly (PI. I, fig. 2), the nasals are seen to be broad and slightly 
arched, increasing in width posteriorly, and extending as far back as the 
anterior margin of the orbit. The relation existing between frontals and 
Fig. 14. 
m s 
Hegetotheriu m mirabile, 
occiput showing the sutures 
between the cranial ele- 
ments, x f (No. 9223, 
American Museum). 
am, auditory meatus ; c, 
condyle ; mp, mastoideo- 
posttympanic suture ; ms, 
mastoideo -supraoccipital 
suture ; pp, posttympanic- 
paroccipital suture. Ter- 
minology after Roth (1903) 
