Sinclair: marsupialia of the santa cruz beds. 351 
specimens, but they were probably as long proportionately as in Thyla- 
cynus. The ventral surface of the centrum (PL LII, fig. 6) has a strong 
median keel which increases in depth posteriorly. The concavities on 
either side are bounded externally by the inferior edges of the transverse 
processes. Traces of the suture uniting the axial centrum with the 
anterior cotyles and odontoid are visible in both species. 
The neural spine of the third cervical is proportionately larger and the 
median keel of the centrum stronger than in Thylacynus. In the fourth 
and fifth cervicals the diapophyses are well differentiated from the inferior 
lamella, unlike Thylacynus . The inferior lamella of the sixth cervical is 
less elongated antero-posteriorly than in the recent genus, but is much 
deeper. The transverse process of the seventh cervical is perforated by 
the vertebral artery. The neural spines of the cervicals increase in size 
and probably also in height posteriorly. The inferior keels on the centra 
decrease in depth on the fifth, sixth and seventh cervicals. The lateral 
surface of the neural arch above the canal for the vertebral artery is per- 
forated by a small foramen in the second to the seventh cervicals. 
The dorsal vertebras associated with the skeleton of B. tuberata are 
larger than in Thylacynus , with heavier neural spines. The centrum of 
the first is keeled inferiorly, but keels are absent in the vertebrae inter- 
preted as the seventh (PL XLV, fig. 6) and eighth dorsals. The dorso- 
lumbar vertebral formula was probably the same as in Thylacynus and 
has been so represented in the restoration (PL LXI, fig. 3). 
Three caudals are preserved with the skeleton of B. tuberata , which are 
interpreted as the third, fourth and fifth. The fourth caudal (PL LIII, 
figs- 9> 9 a ) h as been selected for illustration owing to its better state of 
preservation. It is considerably larger than the corresponding vertebra 
in Thylacynus , but much smaller than the fourth caudal in Prothylacynus . 
The transverse processes are broadly expanded, but are rounded at the 
tips, in contrast with the antero-posterior extension of the tips of the trans- 
verse processes of the proximal caudals in Thylacynus. The presence of 
chevrons on the fourth and fifth caudals is indicated by facets. 
The floor of the neural canal in all the cervicals, except the atlas, and 
in the dorsal and anterior caudal vertebrae is subdivided by a median 
ridge, on either side of which a foramen pierces the centrum. A similar 
structure is observable in Thylacynus and also in various placental carni- 
vores. 
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