74 
PATAGONIAN EXPEDITIONS : PALEONTOLOGY. 
contact with the frontal (text fig. 16), from which it is excluded in Proty- 
potherium by the alisphenoid and parietal, and a second process forward 
and downward, forming the postero-external margin of the alisphenoid 
plate. The bullae are large, heart-shaped, hollow structures, convex in 
all dimensions and tapering to a 
point anteriorly. The auditory 
meatus is long and tubular, with 
its orifice opening posteriorly. It 
is frequently fused with the squa- 
mosal. The basioccipital is sharply 
keeled for a short distance anterior 
to the foramen magnum. 
The distribution of the cranial 
foramina differs in several essential 
particulars from their arrangement 
in Protypotherium and Inter athe- 
rimn (cf. Pis. I, fig. 3 ; III, fig. 3). 
The carotid foramen perforates the 
suture between the basioccipital and 
the tympanic opposite the middle 
of the bulla and is widely separated 
from the foramen lacerum posterius, 
with which it is confluent in Protypotherium and Inter atherium. The 
latter foramen is situated between the basioccipital and the tympanic at 
the inner anterior margin of the base of the paroccipital process. On the 
external side of this process is a deep pit for the tip of the stylohyal. Be- 
tween the auditory meatus and the mastoid are three or four foramina 
apparently opening into lateral vascular sinuses (text fig. 16). A large 
postglenoid foramen perforates the suture between the squamosal and the 
auditory meatus at the margin of the postglenoid fossa. The foramen ovale 
and the foramen lacerum medium are confluent. The Eustachian canal 
opens on the suture between the tympanic and the basisphenoid at the 
elongated anterior extremity of the bulla. Within the orbit a large, elon- 
gated orbito-nasal vacuity, communicating with the infraorbital foramen, 
opens into the olfactory chamber. Several smaller foramina (text fig. 16) 
are probably vascular. The foramen rotundum and spheno-orbital fora- 
men are confluent. A large spheno-palatine foramen connects the spheno- 
maxillary fossa with the posterior narial cavity. 
Fig. 16. 
Hegetotherium mirabile, arch removed to show 
the arrangement of the cranial elements in the 
temporal fossa, Xy (No. 9223, American Mu- 
seum). am, auditory meatus , as, alisphenoid ; 
b, bulla ; c, condyle ; f, frontal ; in, mastoid ; o, 
optic foramen ; os, orbitosphenoid ; p, parietal ; 
pi, palatine ; jt, squamosal. The pterygoid has 
been broken off. 
