300 



first pair of ribs join the produced anterior angles of the manubrium. In the Dasyures, 

 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 Kangaroos have a similar connexion. Four sternebers and a xiphi- 

 sternal complete the breast-bone. 



§ 3. The Pectoral Extremities. — The clavicles are present in all the Marsupials, 

 excepting the Bandicoots (Macrotis, Perameles) and probably also the Choeropus. 

 In the claviculate species they are relatively weakest and shortest in the Kangaroo, 

 strongest and longest in the burrowing Wombat. In the climbing Koala the clavicles 

 are also very strong, but more compressed than in the Wombat, bent outward 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 

 outermost line, which forms the articular surface with the acromion. In 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 Marsupialia. In the Petaurists it forms a 

 scalene triangle, with the glenoid cavity at the convergence of the two longest sides. 

 In the Wombats 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. 



In the Koala the superior costa does not run parallel with the inferior, but recedes 

 from it as it advances forward, 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 

 sigmoid flexure as it approaches the neck of the bone. In the Kangaroos, Bandicoots, 

 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 subscapular surface is remarkable in 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 Kangaroos the supraspinal fossa is of less extent than the space below the 

 spine, and the spine is inclined upward. In the Bandicoots and Dasyures the propor- 

 tions and the supra- and infraspinal surfaces are reversed, and the whole spine is bent 

 downward over the infraspinal surface. In the Potoroos and Phalangers the acromion 

 is, as it were, bent downward, so as to present a flattened surface to the observer ; in 

 the Potoroos and Opossums this appearance is produced by a true expansion of the 

 acromion. In the Bandicoots 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 



