Sinclair: typotheria of the santa cruz beds. 
23 
angular portion of the mandible does not project below this convexity. 
Posteriorly, the angle extends far beyond the condyle. The free border 
is strongly curved inward, inclosing a deep submaxillary fossa. The 
masseteric fossa is shallow, but its boundary is well defined and almost 
circular in outline interiorly. The coronoid process is high, thin and 
sharply pointed, projecting far above the condyle. Its anterior border is 
S-shaped, and strongly inclined forward interiorly. The condyles are 
broadly oval in outline, wider externally than internally and almost flat 
both antero-posteriorly and transversely. The articular surface presents 
upward and forward. Posteriorly, the capitulum supports a non-articular 
projection, which fits into the postglenoid fossa, preventing the backward 
dislocation of the mandible when the mouth is widely opened. A large 
mental foramen is present beneath the fourth premolar or the anterior lobe 
of the first molar, and one or two smaller foramina are in the symphysial 
region beneath the canine. On the inner surface of the ramus, the inferior 
dental canal is circular in outline, with a more or less well defined groove 
leading into it from above, a structure far more strikingly developed in 
Hegetotherium . 
Vertebral Column, Ribs and Sternum . — -The atlas (PI. V, figs. 18-20) 
is characterized by broad transverse processes, with irregularly lobate free 
border, but little basal constriction and no canal for the vertebral artery. 
The neuro-arterial foramina are large and completely inclosed anteriorly 
by strong bony bars. The neural arch is wide, with a large median 
tubercle at its anterior margin. The narrow inferior arch also supports a 
tubercle at its anterior margin, but a much smaller one than that on the 
arch above. 
The axis (PI. V, figs. 16, 17) may be readily recognized by the small 
size of the arterial canal perforating the base of the transverse process. 
The neural spine is strong and hatchet-shaped, projecting beyond the 
centrum posteriorly, where it terminates in a point. This has been broken 
off in the specimen figured. The odontoid is flattened dorso-ventrally, 
its superior surface being flush with the floor of the neural canal, unlike 
Hegetotherium. Interiorly, the centrum is keeled and supports a pair of 
tubercles at its posterior margin. 
The centra of the third and fourth cervicals are also keeled, with similar 
inferior tubercles, but both keels and tubercles are absent from the posterior 
members of the series. The neural spines and transverse processes have 
