MARSUPIALIA. 



281 



Scapula of Koala. 



Fig. 106. curve, the convex pos- 



terior position of which 

 varies as to its degree and 

 extent. 



The subscapular sur- 

 face is remarkable 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 or undulat- 

 ing. 



In the Great Kangaroo 

 the supra-spinal fossa is of 

 less extent than the space 

 below the spine, and 

 the spine is inclined up- 

 wards. In the Perameles and Dasyures the 

 proportions of the supra and infraspinal sur- 

 faces are reserved, and the whole spine is 

 bent downwards over the infraspinal 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 appear- 

 ance is produced by a true expansion of the 

 acromion. In the Perameles the coracoid 

 process is merely represented by a slight pro- 

 duction of the superior part of the glenoid ca- 

 vity. In the Kangaroo and Potoroo it forms a 

 protuberance on the upper part of the head of 

 the scapula. In the other Marsupials it as- 

 sumes the character of a distinct process from 

 the same part, and attains its greatest develop- 

 ment in the Wombat and Koala, in the latter 

 of which it is forcibly curved downwards and 

 inwards. 



The clavicles are present in all the Marsu- 

 pials, with the exception of the genus Pera- 

 meles, and probably also the Charopus. In 

 the claviculate Marsupials they are relatively 

 strongest and longest in the burrowing Wom- 

 bat, weakest and shortest in the Great Kanga- 

 roo. 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 truncate at the sternal extremity, 

 where the articular surface presents a remark- 

 ably deep notch : they become compressed as 

 they approach the acromion, to which they are 

 attached by an extended narrow articular surface. 

 In the Koala the clavicles are also very 

 strong, but more compressed than in the Wom- 

 bat, 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 outermost 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 humerus in the Dasyures and Thyla- 

 cines resembles that of the Dog-tribe in the 



imperforate condition of the inner condyle, but 

 differs in the more marked development of the 

 muscular ridges, especially of that which ex- 

 tends 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. 



In all the other genera of Marsupials that I 

 have examined the internal condyle of the hu- 

 merus is perforated. But in some species of 

 Petaurus, as Petaitrm sciurcus, the foramen is 

 represented by a deep notch ; and in the Plia- 

 lunglstu Cookii, both foramen and notch are 

 wanting.* The ridge above the external condyle 

 is much developed in the Petuurtis mucrurus 

 and aciureus, and notched at its upper part, but 

 this notch does not exist in Pet. taguanoides. I 

 find similar differences in the development of 

 the supinator, or outer ridge, in the genus 

 Perameles; in the Per. lagutis it is bounded 

 above by a groove ; in Per. Gunnii it is less 

 developed and less denned. In the Kanga- 

 roos, Potoroos, Wombat, and Koala (J>g. 107), 

 the outer condyloid ridge extends in the form 

 of a hooked process above the groove of the 

 radial nerve. In all these, and especially in 

 the Wombat, the deltoid process of the hu- 

 merus is strongly developed ; it is continued 

 from the external tuberosity down the upper 

 half of the humerus ; except in the Petaurists, 

 where, from the greater relative length of the 

 humerus, it is limited to the upper third. 



The interspace of the condyles is occasion- 

 ally perforated, as in the Perameles Ingot is and 

 Wombat. The articular surfaces at both ex- 

 tremities of the humerus have the usual form; 

 but it may be observed in some 

 Marsupials, as the Koala, that 

 at the distal articulation the 

 external convexity for the radius 

 has a greater relative extent 

 than usual, and the ulnar con- 

 cavity is less deep. 



The bones of the fore-arm 

 present little to detain our 

 notice. They are always dis- 

 tinct and well developed, and 

 their adaption topronation and 

 supination is complete. The 

 prehensile faculty and ungui- 

 culate structure of the anterior 

 extremities appear to have 

 been indispensable to animals 

 where various manipulations 

 were required in the economy 

 of the marsupial pouch. When, 

 therefore, such an animal is 

 destined like the ruminant to 

 range the wilderness in quest 

 of pasturage, the requisite 

 powers of the anterior members are re- 

 tained and secured to it, as has been already 

 observed, by an enormous developement of the 

 hinder extremities, to which the function of 

 locomotion is restricted. 



* In the other species of Phalangista, and in the 

 Petawrus taguanoides and macrurus, the internal con- 

 dyle of the humerus is perforated. 



Fig. 107. 



Humerus of the 

 Koala. 



