The true or acromial clavicle (fig. 173, A, In yr) is a long, slender, compressed, slightly bent bone, continued from the articular cavity at the end of the acromion to near the median line of the episternum, anterior to but parallel and in contact with the branches of the epi sternum, with which the clavicles finally co alesce, but at an earlier period in the Orni thorhynchus than in the Echidna. These clavicles are the homologues of the os fur catorium in the bird : the T-shaped episternum (fig. 173, a, t) is feebly represented in birds by the median process continued forwards be tween the coracoid articular cavities from the fore part of the sternum. It is in Lizards, and especially in the extinct Ichthyosaur, that the episternum presents the same form, develop ment, and relation to the clavicles, as in the Monotremes. The epicoracoids again are want ing in the bird, but they are present in lizards, and the remarkable breadth of the coracoid in the Enaliosauria is due to their presence, al though, singularly enough, they are anchylosed to the coracoids in these extinct reptiles, while in the warm-blooded Monotremes they remain separate. In the Echidna they are articulated with the coracoid by a true synovial joint. To render the resemblance between the Mono treme and the Bird complete, in respect of the structure of the scapular arch, the episternum must be reduced to a short and simple process attached to the anterior part of the manubrium sterni, the epicoracoids must be removed, and the clavicles anchylosed together at their mesial extremities.
The humerus is a short and strong bone, expanded at both extremities, and, as it were, twisted half round upon itself. The proximal expansion terminates by a broad thick convex border, the middle part of which is developed into the articular head, which is so adapted to the glenoid cavity, that the bone is maintained in a horizontal position, and the distal expan sion is nearly vertical. The deltoid and pec toral crests are strongly developed ; both con dyles are remarkably produced, especially the internal one, which is perforated. (Fig.173, xi, a.) The distal articular surface scarcely occupies a fourth part of that broad termination of the humerus : it presents, in the Echidna (fig. 168), a convex tubercle, which is broadest in front, for the articulation of the radius, nar row behind, for that of the ulna. The articular surfaces of both anti-brachial bones are concave : so that the elbow joint admits freely of flexion and extension, abduction and adduction, but is restricted in the movement of rotation.
The radius and ulna are in contact and pretty firmly connected together through nearly their whole extent; the interosseous space being reduced to a slight fissure. The ulna is chiefly remarkable for the olecranon, (fig. 173, A, u,) which is bent forwards upon the humerus, and transversely expanded at its extremity, especially in the Ornithorhynchus, in which the lower or inner angle of the expanded ex tremity is considerably produced. The shaft
of the ulna is compressed, and increases in breadth, in the Echidna, as it approaches the broad carpus. In the Ornithorhynchus it is bent like the italic,/ is more cylindrical, and more suddenly expanded at the distal end. The radius offers little worthy of notice, except that in the Ornithorhynchus it is flattened next the ulna, and so applied to that bone as to prevent altogether a rotation of the hand upon the ulna. In the Echidna the distal articular surface of the ulna (fig. 175, n) presents two convex trochlea= separated by a median con cavity ; that of the radius (fig. 175, r) offers a reverse condition : here two concavities are divided by a median convex ridge; all the four facets at the carpal joint of the antibrachium are in the same transverse line. The two radial concavities receive the two articular convexities of the broad scapho-lunar bone (fig. 176, a): the two convex trochlew of the ulna play upon two concavities, one-half of each of which is contributed by the cuneiform (fig.176, 6) and pisiform bones (c). This com plicated joint limits the movement of the hand upon the fore-arm to flexion and extension.
Notwithstanding the confluence of the sca phoid with the lunar bone in the carpus of the Echidna, as in that of the Marsupials and Carnivora, it includes eight ossicles, a small sesa moid bone (fig.176, ) being developed in the tendon of the flexor carpi radialis, and articu lated with the scapho-lunar bone and radius. The distal series of the carpus includes the four normal bones, the trapezium (fig. 176, h) supporting the innermost digit or polies, the trapezoides (g), the index, the os magnum (I), which is almost the smallest, sustaining the medius, and the unciforme (e) the two outer digits: this description is taken from the Echidna : the only essential difference obser vable in the Ornithorhynchus is the contribu tion by the os magnum of a greater share to the articulation with the ring-finger.
In the Echidna all the bones of the fore extremity are relatively larger and stronger than in the Ornithurhynchus, but this dif ference is especially remarkable in the meta carpal bones and two first rows of phalanges fig. 176, h, i, k), which are singularly short, broad, and thick. The palm is strengthened by two large sesamoids developed in the flexor tendons in the Echidna ; these are sometimes confluent (fig. 175, /). The number of pha langes in both Monotremes is the same as in other Mammals, viz. two to the thumb and three to each of the fingers. This is not the case in any Saurian, and the retention of the Mammalian type at the peripheral segment of the limb, with the singular deviation from it at the central supporting arch, is not one of the least remarkable points in the osteology of the Monotremes.