350 PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 
foramina are bisected, as in Thylacynus , by the premaxillo-maxillary 
suture. A pair of large foramina pierce the palate opposite the posterior 
margins of the canines. The neuro-vascular foramina at the posterior 
palatal border are enclosed by robust bars of bone, which are much more 
attenuated and often incomplete in Thylacynus ( cf. PI. LXV, fig. i a). 
The narial border is thickened, much as in the creodont Mesonyx. 
The mandible (Pis. XL; XLV, fig. 3) is very heavy in proportion to 
its length. The rami are firmly coossified at the symphysis, but traces of 
the suture remain. The backward inclination of the coronoid process is 
about the same as in Prothy lacy mis, but the width is relatively less. The 
masseteric fossa is broader than in Thylacynus and the heavy flange 
bordering it interiorly is produced to the outer extremity of the condyle, 
while in the latter genus this structure narrows abruptly just anterior to 
the condyle. The condyles are very wide transversely, decreasing in 
width toward their outer ends, while the reverse is true in Thylacynus. 
The angle is broad and less deeply notched posteriorly than in the latter. 
Six mental foramina are present in B. tuberata , varying in position on 
opposite halves of the same mandible. The largest of these is situated 
beneath the anterior premolar. 
Vertebral Column ; Ribs and Sternum. — The atlas (PI. LIII, figs. 2, 2 a, 
4-4 b) is peculiar in lacking a foramen for the vertebral artery and first 
pair of spinal nerves, resembling in this respect Phascolomys. The nerve 
and artery are transmitted through a pair of deep notches in the anterior 
margin of the neural arch. The canal for the vertebral artery is small, 
entering the neural arch just above the condyles and emerging on the 
lower surface of the transverse process. A small foramen, possibly for a 
recurrent branch of the same artery, perforates the upper surface of the 
transverse process near its posterior border. The intercentrum (PI. LIII, 
fig. 4 b) is separately ossified and unfused with the base of the arch. Its 
posterior border supports a small median styloid process. The transverse 
processes are semicircular in outline with thickened edges. 
The axis (PL LII, figs. 1, 2, 6) carries a large hatchet-shaped neural 
spine which overhangs the odontoid anteriorly. Posteriorly, the spine is 
extended to about the same degree as in Thylacynus. The odontoid 
tapers less than in that genus, retaining about the same width through- 
out. Anteriorly, it is obliquely truncated. The transverse processes are 
perforated by the vertebral artery. Their extremities are broken off in both 
