Sinclair: typotheria of the santa cruz beds. 
i9 
pending more on dimensions than on structural peculiarities. In side view 
(PI. Ill, figs. 1, 5, 6), the upper profile of the skull slopes forward gradu- 
ally from the elevated parietal region, with a more abrupt slope back- 
ward, especially in P. attenuatum , in which the parietal tract is consider- 
ably elevated and globose. The orbits are central and widely open 
posteriorly, of moderate size and less prominent than in Inter atherium, 
Hegetotherium or Pachyrukhos. The rostrum is laterally compressed and 
excavated longitudinally. The facial portion of the premaxillary is broad, 
with short ascending process and attenuated anterior extremity, terminat- 
ing in a prominent anterior nasal spine. On the palatal surface, the incis- 
ive foramina are confined almost entirely to the premaxillae, touching the 
maxillae posteriorly along the line of the premaxillo-maxillary suture. The 
canine is implanted entirely in the maxillary, the suture between the pre- 
maxillary and the maxillary passing between the third incisor and the 
canine. The maxillary forms the entire lower surface of the zygomatic 
arch, supporting anteriorly a small descending process. It extends as far 
back as the posterior border of the glenoid cavity and forms with the 
malar a prominent buttress preventing the lateral displacement of the 
lower jaw. The maxillary enters also into the anterior and inferior bound- 
ary of the orbit. Its ascending process is acute and greatly prolonged, 
extending almost as far as the middle of the orbit, where it is received 
in a deep notch in the frontal. The facial expanse of the maxillary is 
broadly excavated horizontally. At the posterior margin of this excava- 
tion, the large infraorbital foramen perforates the base of the elevated orbi- 
tal rim. Neither maxilla nor premaxilla is firmly united with the nasals, 
which, owing to a lack of support, are frequently crushed down into the 
narial chamber. The lachrymal is scale-like, with but little facial expan- 
sion. It supports a prominent tubercle, beneath which, on the orbital 
surface, is the lachrymal duct. A small “ lachrymal ” tubercle is present 
also on the maxillary (PI. Ill, fig. 5). The malar is a long, narrow ele- 
ment confined entirely to the zygomatic arch, where it is supported beneath 
by the maxillary and is overlapped above by the temporal process of the 
squamosal. Anteriorly, it touches the orbital rim, while posteriorly it ex- 
tends slightly beyond the maxillary process. The root of the temporal 
process of the squamosal and a large part of the cranial expanse of the 
same element are dilated and filled with cancellae (PI. Ill, fig. 7), the 
chambers communicating with each other as well as with the tympanic 
