Of the Pectoral Extremities.—The scapula varies in form in the different Marsupials. 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 re gular 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 inch above the plane of the scapula, and terminates in a long narrow compressed acroniion arching over the neck to reach the clavicle.
In the Koala (fig. 106), the superior costa does not run parallel with the inferior, but re cedes 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 Potoroo, 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 scapula.
In the Great Kangaroo, the Perameles, langers, Opossums, and Dasyures, the whole upper costa of the scapula describes a sigmoid 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 Chtsropus. 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 cincs 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.