400 PROF. OWEN ON THE OSTEOLOGY 
inch above the plane of the scapula, and terminates in a long, narrow, compressed 
acromion, arching over the neck to meet the clavicle. 
In the Koala the superior costa does not run parallel with the inferior, but recedes 
from it as it advances forwards, and then passes down, forming an obtuse angle, and 
with a gentle concave curvature, to the neck of the scapula ; a small process extends 
from the middle of this curvature. In the Potoroos the upper costa is at first parallel 
with the lower, but this parallel part is much shorter ; the remainder describes a sig- 
moid flexure as it approaches the neck of the scapula. In the Great Kangaroo, the 
Perameles, Phalangers, Opossums and Dasyures, the whole upper costa of the scapula 
describes a sigmoid curve, the convex posterior portion of which varies as to its degree 
and extent. 
The sub-scapular surface is remakable in the Perameles for its flatness, but presents 
a shallow groove near the inferior costa. In most other Marsupials it is more or less 
convex and undulating. 
In the Great Kangaroo the supra-spinal fossa is of less extent than the space below 
the spine, and the spine is inclined upwards. In the Perameles and Dasyures the pro- 
portions of the supra- and infra-spinal surfaces are reversed, and the whole spine is 
bent downwards over the infra-spinal surface. In the Potoroos and Phalangers the 
acromion is, as it were, bent downwards, so as to present a flattened surface to the 
observer ; in the Potoroos and Opossums this appearance is produced by a true expan- 
sion of the acromion. In the Perameles the coracoid process is merely represented by 
a slight production of the superior part of the glenoid cavity. In the Kangaroos and 
Potoroos it forms a protuberance on the upper part of the head of the scapula. In the 
other Marsupiata it assumes the character of a distinct process from the same part, 
and attains its greatest development in the Wombat and Koala, in the latter of which 
it is forcibly curved downwards and inwards. 
The humerus in the Dasyures and Thylacine resembles that of the Dog-tribe in the 
imperforate condition of the inner condyle, but differs in the more marked develop- 
ment of the muscular ridges, especially of that which extends upwards from the outer 
condyle for the origin of the great supinator muscle ; this ridge is terminated abruptly 
by the smooth tract for the passage of the musculo-spiral nerve. Jn all the other genera 
of Marsupials that I have examined the internal condyle of the humerus is perforated. 
It is so in the Petawrus macrourus and Pet. Taguanoides, but in the Petaurus sciureus 
the foramen is represented by a deep notch; in the Phalangista Cookw both foramen 
and notch are wanting. The ridge above the external condyle is much developed in 
the Petaurus macrourus and sciureus, and notched at its upper part ; but this notch does 
not exist in Pet. Taguanoides. I find similar differences in the development of the su- 
pinator or outer ridge in the genus Perameles. In the P. nasuta it is bounded above 
by a groove ; in a smaller species it is less developed and less defined. In the Kanga- 
roo0s, Potoroos, Wombat and Koala the outer condyloid ridge extends in the form of a 
