52 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
margin of the ascending ramus is sigmoid in outline, as in Protypotherium. 
The sigmoid notch is broad and the condyle irregularly elliptical in out- 
line, broader externally than internally, with its margin overhanging the 
neck. The groove terminating in the inferior dental canal is much longer 
than in Protypotherium. Two mental foramina are present, one beneath 
the canine and the other beneath the last premolar, or the anterior part of 
the first molar. Sometimes the latter foramen is doubled. The masse- 
teric fossa is almost circular, as in Protypotherium , but less sharply defined 
anteriorly and with proportionately stronger transverse ridges for muscu- 
lar attachment. 
Vertebral Column; Ribs and Sternum. — The atlas (PI. VIII, figs. 13- 
15, 28) may be readily recognized by the prominence of the neural spine, 
the robustness of the transverse processes and the position of the canal for 
the vertebral artery. The latter perforates the base of the transverse 
process at the margin of the posterior cotylar surface, emerging on the 
lower surface of the process near its anterior margin. Between the point 
of emergence and the neuro-arterial canal, the artery lay in a groove 
(sometimes enclosed as a foramen, PI. VIII, fig. 13) between the trans- 
verse process and the anterior cotylus. The neuro-arterial canals are 
large and are inclosed anteriorly by robust bony bars. The transverse 
processes vary somewhat in shape and degree of expansion. In I. robus- 
tum and /. extension , there is but little basal constriction, while the free 
border is broadly expanded, especially antero-externally. In I. excava- 
tion , there is considerable basal constriction and the free border is less 
expanded (PI. VIII, fig. 28). The anterior margin of the neural arch 
supports a large and very prominent spine-like tubercle. A smaller 
sharp-pointed spine projects backward from the posterior margin of the 
inferior arch. 
The axis (PI. VIII, figs. 11, 12) is characterized by a large, hatchet- 
shaped neural spine, which varies slightly in shape in specimens referred 
to one and the same genus. In some, it is strongly convex dorsally, with 
round anterior and posterior extremities. In others, the dorsal margin is 
horizontal or nearly so. The transverse processes are slender, sharp- 
pointed and directed posteriorly. Their bases are perforated by the canal 
for the vertebral artery. The odontoid is short and robust, its dorsal 
surface lying in the same plane as the floor of the neural canal. The 
centrum is strongly keeled interiorly, the keel bifurcating posteriorly. The 
