324 ANATOMY- OF VERTEBRATES. 



of the manubrium stferni. The posterior margin of the coracoid 

 is concave and free; thfe'an^rior- margin is straight and articu- 

 lated with a narrower ' epicoracoid ' in the Echidna than in the 

 Ornithorhynchus. 



The clavicles, fig. 199, m, m, are two curved styles, extending 

 from the acromion along the transverse bar of the episternum, t. 

 The humerus, ib. 53, is remarkable for its shortness and breadth, 

 especially of its two extremities. There is a small sesamoid os- 

 sicle, above the internal tuberosity, answering to the ' os humero- 

 capsulare ' in the shoulder-joint of Birds (p. 67). 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 expansion is nearly vertical. 

 The deltoid and pectoral crests are strongly developed ; both 

 condyles are remarkably produced, especially the internal one, 

 which is perforated, fig. 199, H, a. The distal articular surface 

 scarcely occupies a fourth part of that broad termination of the 

 humerus: it presents, in the Echidna, fig. 201, 5.3, a convex 

 tubercle, which is broadest in front for the articulation of the 

 radius, narrow behind for that of the ulna. The articular sur- 

 faces of both antibrachial bones are concave : so that the elbow- 

 joint admits freely of flexion and extension, abduction and adduc- 

 'tion, but is restricted in the movement of rotation. 



The radius, fig. 199, 5 4, and ulna, ib. 55, are in contact and 

 pretty firmly connected together through nearly their whole ex- 

 tent; the interosseous space being reduced to a slight fissure. 

 The ulna is chiefly remarkable for the olecranon, fig. 199, I, u, 

 which is bent forward upon the humerus, and transversely ex- 

 panded at its extremity, especially in the Ornithorhynchus, in 

 which the lower or inner angle of the expanded extremity is con- 

 siderably 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 f, is 

 more cylindrical, and more suddenly expanded at the distal end. 

 The radius offers little worthy of notice, except that in the Orni- 

 thorhynchus 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. 207, n, presents two convex trochlege separated by a median 

 concavity ; that of the radius, ib. r, oflPers 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 



