MONOT11EMATA. 



377 



rower corresponding hone in the Echidna. 

 These hones, which are called the ' epicoracoids' 

 (Jig- 173, A, n n) are joined by their median 

 margin to the stem of the episternal, and by 

 their anterior margins to its transverse branches, 

 which are overlapped by the epicoracoids. 



The true or acromial clavicle (fig. 173, A, 

 m /) 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 witli 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, 

 H, a.) The distal articular surface scarcely 

 occupies a fourth part of that broad termination 

 of the humerus : it presents, in the Echidna 

 (Jig- 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. 



* For a full and elaborate discussion of the 

 various opinions which have been offered respect- 

 ing the homology or signification of the complicated 

 apparatus of the shoulder in the Ornithorhynchus, 

 the reader is referred to Meckel, ' De Oruithorhyn- 

 cho/ &c. pp. 12-15. 



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, (Jig. 173, 

 A, M,) 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 italicyi 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 (Jig. 175, n) presents two 

 convex trochleae separated by a median con- 

 cavity ; that of the radius (Jig. 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 

 (Jig. 176, ): the two convex trochleae of the 

 ulna play upon two concavities, one-half of 

 each of which is contributed by the cuneiform 

 (Jig. 176, 6) and pisiform bones (c). This com- 

 plicated joint limits the movement of the hand 

 upon the fore-arm to flexion and extension. 



Fig. 175. 



Bones of fore-foot, palmar aspect, Echidna setosa, 

 ( Original.) 



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 (Jig. 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 {'Jig. 176, /<) 



