366 PATAGONIAN EXPEDITIONS: PALAEONTOLOGY. 
The seventh dorsal, if the position of the vertebra represented in fig. 4, 
PI. LI, has been correctly interpreted, is of about the same size as the 
corresponding vertebra in Thylacynus, but is considerably smaller than 
the seventh dorsal in Borhycena (PI. XLV, fig. 6). The neural spine is 
much wider transversely than in the former genus. The centrum is with- 
out inferior keel. The tenth dorsal, or anticlinal vertebra, (PI. LIII, fig. 
7) lacks the tip of the neural spine, the anterior margin of which slopes 
steeply forward, while the posterior margin descends vertically. Promi- 
nent metapophyses and anapophyses are developed on the tenth dorsal 
and also on the succeeding dorsals, so far as preserved (PI. LXI, fig. 2). 
The centra of the tenth and eleventh dorsals are without inferior keels. 
The centrum of the twelfth is slightly keeled. 
The dorso-lumbar formula was probably the same as in Thylacynus — 
thirteen dorsals and six lumbars. Three of the latter are preserved (PI. 
LI, fig. 5). The centra are wider posteriorly than anteriorly, producing 
an hour-glass shape. Unlike Thylacynus , they are not keeled inferiorly. 
As in that genus, the lower surface of each centrum is perforated by one 
or more large foramina. The neural spines, although incompletely pre- 
served, are seen to be much heavier than in the recent genus. Meta- 
pophyses are less strongly developed than in the posterior dorsals. The 
anapophyses decrease regularly in size, until, in the sixth lumbar they are 
mere points. The transverse processes are heavier than in Thylacynus , 
and, as in that genus, curve forward and downward. The sides of the 
neural arches are pierced by small foramina. 
Two vertebrae are coossified in the sacrum (PI. L, figs. 3, 3 a) which 
differs from that of Thylacynus ( cf text-fig. 3, b , c) in the greater size 
and more complete fusion of the sacral elements. The dorsal interverte- 
bral fenestra, present in the latter genus between the first and second 
sacrals, is wanting in Prothylacynus. 
The proximal caudals (PI. LIII, fig. 8) are much larger than in either 
Thylacynus or Borhycena. The caudal centra rapidly increase in length, 
retaining functional postzygapophyses farther back in the series than in 
Thylacynus. The transverse processes have been more or less broken 
from all the caudals preserved, but were evidently very heavy (PI. LIII, 
figs. 6, 8). Facets for chevrons are present on all the caudals, beginning 
with the third. The proximal caudals are unkeeled. The seventh, 
eighth and ninth are doubly keeled inferiorly. Judging from the size of 
