350 PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 



foramina are bisected, as in Thy lacy mis, 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 Thylacymis (cf. PI. LXV, fig. ia). 

 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 Prothylacynus, but the width is relatively less. The 

 masseteric fossa is broader than in Thylacynus and the heavy flange 

 bordering it inferiorly 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*7, 

 4-46) 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^) 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 (PI. LII, figs, i, 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 TJiylacymis. 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 



