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 Petaurus seiureus, the foramen is represented by a deep notch ; and in the Pha langista Cookii, both foramen and notch are wanting.• The ridge above the external condyle is much developed in the Petaurus macrunis and sciareus, 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. lagbtis it is bounded above by a groove ; in Per. Gunnii it is less developed and less defined. In the Kanga roos, Potoroos, Wombat, and Koala (fig. 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 s 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 to pronation 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 reqnired 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.
We find, therefore, that the bones of the fore arm of the Kangaroo differ little from those of the burrowing Wombat, the climbing Koala, or the carnivorous Dasyure, save in relative size. They present the greatest proportional strength in the Wombat, and the greatest proportional length and slenderness in the Petaurists or Fly ing Opossums, in which the radius and ulna are in close contact through a great portion of their extent, and thus lend a firmer support to the out stretched dermal parachute. They are also long
and slender in the Koala. In general the radius and ulna run nearly parallel, and the interos seous space is very trifling. It is widest in the I'otoroos. The olecranon is well developed in all the Marsupials. In the Virginian Opossum and Petaurists we find it more bent forwards upon the rest of the ulna, than in the other Mar supials. In the Wombat, where the acromion is the strongest, and rises an inch and a half above the articular cavity of the ulna, it is extended in the axis of the bone. The distal end of the radius in this animal is articulated to a bone representing the os scaphoides and os lunare.
The ulna, which in the same animal con verges towards a point at its distal end, has that point received in a depression formed by the cuneiform and pisiform bones ; these are bound together by strong ligaments, and the pisiform then extends downwards and backwards for two-thirds of an inch. The second row of the carpus consists of five bones. The trapezium supports the inner digit, and has a small sesa moid bone articulated to its radial surface. The trapezoides is articulated to the index digit, and is wedged between the scapho-lunar bone and os magnum ; this forms an oblique articu lar surface for the middle digit ; but the largest of the second series of carpal bones is cuneiform, which sends downwards an obtuse rounded process, and receives the articular surface of the fifth, and the outer half of that of the fourth digit, the remainder of which abuts against the oblique proximal extremity of the middle metatarsal bone.
The five metacarpal bones are all thick and short, but chiefly so the outermost. The inner most digit, or pollex, has two phalanges, the remainder three ; the ungueal phalanx of all the digits is conical, curved, convex above, ex panded at the base, and simple at the opposite extremity.
In the Perameles the ungueal phalanx of the three middle digits of the hand, and of the two outer digits of the foot, are split at the extremity by a longitudinal fissure commencing at the upper part of the base. This structure, which characterizes the ungueal phalanges in the Placental Anteaters, has not been hitherto met with in other Marsupial genera.* The terminal phalanges of the Koala are large, much compressed and curved ; the con cave articular surface is not situated, as in the cats, on the lower part of the proximal end, but, as in the sloths, at the upper. The claws which they support are long.