90 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
dental canal, which opens into the masseteric fossa close to the anterior 
border of the latter (PL X, fig. i). The symphysis is firmly coossified 
and spout-like. The horizontal ramus increases rapidly in depth pos- 
teriorly, the coronoid border is strongly inclined backward, and the angle 
broadly rounded. The coronoid process is small, sharp-pointed and quite 
delicate and is accordingly broken in the majority of specimens. The 
condyle is approximately circular in outline and almost flat, present- 
ing upward and forward. The inferior margin of the angle has an out- 
wardly directed, sharp ridge as in Hegetotherium. 
Vertebral Column and Ribs. — The atlas (PI. X, figs. 24-26) is peculiar 
in having the neuro-arterial foramina reduced to notches and in the irregu- 
larly lobate outline of the free borders of the transverse processes. Each 
arch supports a single, prominent, median tubercle. Owing to the broken 
condition of the superior arch and the absence of good contacts, it is not 
possible to be certain whether the tubercle is at the anterior or posterior 
border, but, judging from the structure of this region in Protypotherium 
and Hegetotherium , it was probably anterior. The posterior third of the 
base of the transverse process is perforated by the vertebral artery, as in 
the last named genus. 
The axis is incompletely preserved in one specimen (No. 15,744). As 
in Hegetotherium , the odontoid curves upward sharply. The neural spine 
is almost complete, differing considerably in shape from that of Protypo- 
therium (cf. Pis. V, fig. 1 6 ; X, fig. 23). Anteriorly and posteriorly, it has 
about the same degree of extension and is evenly convex above. Its pos- 
terior margin is grooved below to receive the tip of the neural spine of 
the third cervical, when the neck is flexed upward. This spine is short, 
broad and directed vertically. The spines of the fourth, fifth and sixth 
cervicals are delicate, sharp-pointed, and incline forward. The transverse 
processes are heavy, with prominent anterior and posterior tubercles. 
The differentiation of the diapophysis from the inferior lamella is first 
observable in the fifth cervical. The centra are keeled inferiorly. 
The number of dorsal vertebrae is not positively known, but probably 
is not more than fifteen, as in Inter at herium. Eight lumbars are present 
(seven in Interatherium ) and five vertebrae are coossified in the sacral 
complex, three true sacrals and two caudals. The anterior dorsals are 
either crushed or concealed by other bones which cannot be removed 
without injury to the specimens. The anticlinal vertebra appears to be the 
