Of the pectoral extreinities.—Cuvier* justly observes that the most remarkable part of the osteology of the Monotremes is the organiza tion of the shoulder ; which corresponds with that of birds, and still more with that of lizards.
Had these anomalous Mammals been ex tinct, and their fossilized skeleton alone, as in the Ichthyosauri, been preserved for the con templation of the naturalist, the perplexity which the combination of this structure with the mammalian conditions of the skull and vertebra would have occasioned may be readily conceived.
The scapula (/) is represented detached, with the corticoid (o), at o, fig. 173 ; /* is the cartilage appended to the short convex base. The scapula is long, narrower than in most AIammalia, and has its posterior vertebral angle so much produced, as to give it a re semblance to the scapula of the bird and saurian : this resemblance is farther increased by the origin of the spine close to the anterior costa, and by the spine being bent forwards so as to seem to form a continuation of the external surface of the scapula, which is thus rendered concave in the Ornithorhynchus.
The spine, however, terminates in a freely pro jecting acromion.
The true anterior costa is, in this Monotreme, represented by a ridge which traverses ob liquely the inner and convex side of the sca from the anterior vertebral angle to the neck of the bone. In the Echidna this ridge is nearly obsolete, and the spine of the sca pula is bent so as to form a more direct con tinuation of body of the scapula with the plane of which it is nearly parallel : the acromial ter mination is slightly twisted.
Both Cuvier and Meckel describe the spi nous process of the scapula as the anterior margin, (superior costa in human anatomy,) and consequently consider the spine of the scapula as being absent. Cuvier says, " Le bord ant6rieure descend presque droit jusqu'a l'endroit ou it se courbe en dedans pour former une apophyse, qui porte la fourchette.' Meckel recognises this process as the acromion :" Margo anterior partis superioris versus inferiora ex trorsum primo flexus, dein eminentiam, acro mion, antrorsum et introrsum versam, emittit." The ' margo anterior' of Meckel, bord ante rieure' of Cuvier, is, in fact, the true spine of the scapula, and the true margo anterior' is the ridge above described. The proof of this
is afforded by the origin of the supra-spinatus muscle which occupies the space between Meckel's margo anterior' and the ridge which I regard as the true anterior costa, and which is not noticed by either of the anato mists above quoted.
Since the scapula is peculiarly characterized in Mammalia by the presence of a spine and in Ovipara by its absence, its recognition in the Monotremes, under the modification by which its apparent absence is occasioned, and the transition to the oviparous type of this bone is effected, becomes a subject of especial in terest.
The whole scapula is broader, thicker, and less curved in the Echidna than in the Omi thorhynchus. In both Monotremes, the pos terior margin or costa is concave, most so in the Ornithorhynchus, and in both it is turned towards the trunk, so that the sub-scapular surface looks obliquely forwards and inwards. The articular surface is divided into two facets: the one, internal and flat, articulates with the corticoid ; the other, external, is slightly con cave, and contributes, with a similar but nar rower concave surface of the coracoid, to form the glenoid cavity for the humerus.
The coracoid (jig. 173, c, o) early coalesces with the scapula in the Ornithorhynchus; it main tains its independent condition to a later period in the Echidna. In both it is a strong, subcom pressed, subelongate bone, expanded at both ends; one of these is articulated and anchylosed with the scapula, as above described; the other is joined to the anterior and external facet of the manubrium sterni. The posterior margin of the corticoid is concave and free; the anterior margin is straight and articulated with a thin broad irregularly quadrilateral plate of bone in the Ornithorhynchus, and a thicker and nar rower corresponding hone in the Echidna. These bones, which are called the' epicoracoids' (fig. 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.