OF THE MARSUPIALIA. 399 
join it, nor are they confluent, but have a gliding motion one over the other. In the 
Opossum there are seven pairs of true ribs, and six which may be regarded as coste 
nothe. In the Petaurists six pairs out of the twelve, and in the Wombat six pairs only 
out of the fifteen reach the sternum. 
The sternum consists of a succession of elongated bones, generally six in number, 
but in the Petawrus Taguanoides five, and in the Wombat four. The first bone, or manu- 
brium sterni, is the largest, and presents in many species a triangular shape, from the 
expansion of its anterior part, and sometimes a rhomboidal figure. A strong keel or 
longitudinal process is given off in many species from the middle of its inferior or outer 
surface ; the side next the cavity of the chest is smooth and slightly concave. In the 
Wombat, Phalangers and others, the keel is produced anteriorly into a strong pro- 
cess, against the sides of which the clavicles abut. The first pair of ribs join the pro- 
duced anterior angles of the manubrium. In the Dasywres, Opossums, Phalangers and 
Petaurists the manubrium is compressed and elongated, and the clavicles are joined 
to a process continued from its anterior extremity. The small clavicles of the Kan- 
garoo have a similar connection. 
The cartilages of the true ribs (which frequently become ossified in old Marsupials) 
are articulated, as usual, to the interspaces of the sternal bones ; the last of these bones 
supports a broad flat cartilage. 
Of the Pectoral Extremities.—The clavicles are present in all the Marsupials excepting 
the Perameles, and probably also the Cheropus. In the claviculate species they are 
relatively strongest and longest in the burrowing Wombat, weakest and shortest in the 
Great Kangaroo. In the latter they are simply curved, with the convexity forwards, 
and measure only two inches in length. In the Wombat they are upwards of three 
inches in length, and have a double curvature ; they are expanded and obliquely trun- 
cate at the sternal extremity, where the articular surface presents a remarkably deep 
notch ; they become compressed as they approach the acromion, to which they are at- 
tached by an extended narrow articular surface. In the Koala the clavicles are also 
very strong, but more compressed than in the Wombat, bent outwards in their whole 
extent, and the convex margin formed, not by a continuous curve, but by three almost 
straight lines with intervening angles, progressively diminishing in extent to the out- 
ermost line, which forms the articular surface with the acromion. In the Myrmecobius 
the clavicles are subcompressed, and more curved at the acromial than at the sternal 
end. In most of the other Marsupials the clavicle is a simple, compressed, elongated 
bone, with one general outward curvature. 
The scapula varies in form in the different Marsupiata. In the Petaurists it forms a 
scalene triangle, with the glenoid cavity at the convergence of the two longest sides. In 
the Wombat it presents a remarkably regular oblong quadrate figure, the neck being 
produced from the lower half of the anterior margin, and the outer surface being 
traversed diagonally by the spine—which, in this species, gradually rises to a full 
VOL. 1J.—PART V. 36 
