486 PROFESSOR OWEN ON THE SPECIES OF PHASCOLOMYS. 
The number of vertebr which are confluent in the sacral region of Phascolomys 
latifrons is four (P]. LXX. fig. 1, s 1-s 4), the two foremost of which articulate with 
the ilia. The syndesmotic surface is formed on each side by an expansion of the hinder 
half of the diapophysis of the first vertebra (ib. d 1) and of the whole of that of the 
second (d 2), forming together an oblong subreniform tract (ib. fig. 2) with an indication 
of its division into two surfaces (d 1, d 2). The entire longitudinal extent of this joint is 
1 inch 2 lines, with an extreme breadth of 6 lines. ‘The diapophyses are confluent along 
their expanded distal halves of the first and second, and by a less proportion in the third 
and fourth sacrals, leaving vacuities (sd 1-sd 3) between their proximal portions, into 
which open the joints between the bodies of the sacral vertebre. These joints are not 
anchylosed in the specimen figured. ‘The bodies of the sacral vertebree (ib. fig. 1, s 1-s 4) 
are depressed, losing vertical extent as they recede, but maintaining breadth beyond the 
first, which is the largest. In the first sacral the prezygapophyses resemble those of 
the lumbar vertebra, and develop external to the joint a small tuberous metapophysis. 
The postzygapophyses are small; and both articular processes of the neural arch 
decrease in size to the last sacral. Between this and the third sacral the coalescence 
is limited to the extreme ends of the diapophyses, which in the last (d 4) are produced 
forward. 
In the first caudal (ib. fig. 1, ¢ 1) the broad depressed diapophyses (d 5) are curved 
backward, as in the succeeding caudals (d 6, d 7). 
In a large full-grown Phascolomys platyrhinus there are four sacral vertebre by ter- 
minal coalescence of diapophyses, the two anterior of which articulate with the ilia, the 
articular surface being extended along the whole terminal expanse of the first sacral 
diapophyses. ‘The fifth vertebra by the backward direction of its diapophysial expan- 
sions indicates its caudal characters; but on the right side the diapophysis is confluent 
with that of the following vertebra. 
In the sacrum of a second, not quite full-grown, example of Phascolomys platyrhinus 
(Pl. LXX. fig. 5) the vertebra (c 1) succeeding the four anchylosed sacrals has its dia- 
pophyses (d 5) similarly directed and expanded, the left touching the one in advance by 
its extreme angle, with the interposed ligamentous matter not yet ossified. In the 
sixth vertebra (¢ 2) the diapophysial expansions extend backward and coalesce at their 
hinder angles with the diapophyses of the seventh vertebra, forming, as it were, a second 
small sacrum (d 6, d 7), according to the character of coalescence. 
The articular surface for the ilium (Pl. LXX. fig. 4) is longitudinally more extended, 
and the proportions contributed by the first and second sacrals (d 1, d 2) are further 
apart than in Phascolomys latifrons (fig. 2). 
In the example of Phascolomys vombatus, described in the ‘ Catalogue of the Osteology 
in the Museum of the Royal College of Surgeons’ (4to, 1853, no. 1814, p. 333), anchy- 
losis of the fifth vertebra having expanded antroverted diapophyses with those of the 
fourth sacral has been completed, and a sacrum of five vertebre by coalescence results. 
