66

It has always a mesosca

the Bandicoots (Peramelidæ). pular segment" as its outer end, and a "precoracoid segment" at its sternal end; these are, however, not ossified. Most generally it is attached to the acromion by a rather strong ligament, but in the Wombat by a synovial articulation. It is usually connected to the presternum by omosternal cartilages of varying length, best developed in the Didelphida.

The shoulder girdle of the MONOTREMATA (see Figs. 84 and 85) differs widely, in many points, from that of any other Mammal, and far more resembles that of the Lizards.

The scapula is rather long and narrow, and (especially in the Ornithorhynchus) curved backwards and pointed, sicklelike, at its upper end. Instead of three it presents but two distinct borders and two surfaces; but the more convex border (s), which is turned forwards and outwards in its natural position, has a small projection (a) near its lower end, which affords attachment to the clavicle, and is evidently the acromion; and the whole border may be considered to represent the spine. Following the indications afforded by the attachment of the muscles, it appears probable that the whole inner surface represents the prescapular fossa of the ordinary Mammalian scapula, and that the anterior portion of the outer surface (pf) is the postcapular fossa, and the posterior portion of the same surface (ssf) the subscapular fossa, these two being divided below by a slight ridge (gb), which runs to the edge of the glenoid cavity, and from which the long head of the triceps muscle takes origin. This ridge then answers to the posterior or glenoid border of the ordinary Mammal, and the hinder border of the Monotreme's scapula (c) would correspond to the anterior or coracoid border. If this is really the case, the scapula of the Monotreme and that of the Cetacean offer the widest contrast,

the supposed primitive trihedral rod being flattened in opposite directions. In the Cetacean scapula there are two nearly parallel surfaces, the postscapular and the subscapular fossa; while the third, the prescapular fossa, is reduced to the smallest possible width-quite obsolete, in fact—in Platanista. In the Monotreme the last-named fossa is so expanded that the other two, instead of being parallel to

FIG. 84. Side view of right shoulder girdle of a young Echidna (Echidna hystrix), ss suprascapular epiphysis; ss subscapular fossa; pf postscapular fossa; co coracoid border; gb glenoid border; s spine; a acromion; css coraco-scapular suture; gc glenoid cavity; c coracoid; ec epicoracoid; c/ clavicle; ic interclavicle ; ps presternum.

each other on opposite sides of the bone, are brought almost into one plane, which is parallel and opposite to the subscapular fossa.

The coracoid (c) is a stout subcylindrical bone, expanded at its extremities, taking at its upper end a considerable share in the formation of the glenoid cavity, and becoming firmly ankylosed with the scapula. At its lower end it

articulates to the side of the presternum, just in front of the first rib.

Placed in front of the inner end of the coracoid is a broad, flat, shield-like plate of bone (epicoracoid, ec Fig. 84, Prc, Fig. 85), the rounded inner border of which passes beyond the median line, overlapping the corresponding bone of the opposite side. In the Echidna the left lies superficially to the right, while in the Ornithorhynchus this disposition is reversed.

Upon the front end of the presternum, lying below its anterior continuation (proosteon, see p. 104) and also below

SC.....

FIG. 85.-Ventral view of shoulder-girdle and anterior portion of sternum and sternal ribs of Ornithorhynchus (O. (anatinus); . Sc scapula; cl calvicle; c coracoid; Prc epicoracoid; Ep interclavicle or episternum.

the epicoracoids, is a large azygous T-shaped bone (ic Figs. 84 and 44; Ep Fig. 85), which has no homologue in any other Mammal, called interclavicle or episternum. Its lower end is broad, and rests on the expanded straight upper margin of the presternum; it contracts somewhat above before dividing into a pair of nearly horizontal, slightly-curved arms, which extend outwards towards, though not quite reaching, the acromion. This bone differs from the presternum, and the small proosteal plate behind its lower

extremity, as well as the coracoids and epicoracoids, in being developed in membrane.

The clavicles (c) are simple, elongated, slightly curved, thin, splint-like bones, resting upon the anterior surface of the arms of the interclavicle, pointed and not quite meeting internally, and dilated and articulating directly with the acromion at their outer end.

CHAPTER XV.

THE ARM AND FORE-ARM.

IN the upper segment of the limb proper there is always one bone, the Humerus; in the second segment, two bones placed side by side, the Radius and Ulna.

The Humerus (except in some of its extreme modifications) is more or less elongated and cylindrical. It is described as having a shaft, and two extremities. The upper or proximal extremity has a smooth, convex, generally more or less rounded head (Fig. 86, h), the axis of which is directed upwards and backwards.1 This, in the living animal, is covered with a thin layer of cartilage, and articulates by a synovial joint with the glenoid cavity of the shoulder girdle. The head is marked off from the shaft very indistinctly by a constriction called the neck, immediately below which, upon the anterior surface of the bone, are two rough prominences ( and t) for the attachment of muscles, called tuberosities, separated from each other by a groove (bg) called the bicipital groove, as the tendon of the biceps muscle runs in it after arising from the margin of the glenoid fossa. The tuberosities are generally distinguished

1 The terms of relative position here used are those which the bone assumes in the ordinary attitude of a quadruped while standing or walking.