In the Amphibia the dorsally-placed scapula (fig. 27, S) has more dorsally still a cartilaginous plate, the supra-scapula (fig. 27, S.S), which may be calcified. The precoracoid (fig. 27, P.C) and coracoid (C) are quite distinct, the former being in front (ceph alad) and overlaid by a dermal bone, the clavicle (Cl). Uniting the ventral ends of the precoracoid and coracoid is the epicoracoid on each side (fig. 27, E.C).
In the Reptilia the same general plan is evident, but in the lizards the ventral ends of the two clavicles are united by a median dagger-like dermal bone, the interclavicle (fig. 27, I.C), which lies or. a plane superficial to the sternum and epicoracoids.
In birds the scapula has the shape of a sabre blade, and there is a rudimentary acromion process, though this is also indicated in some reptiles. The pre- and epi-coracoids are aborted, but the coracoids are very strong. The clavicles and interclavicle unite into a V-shaped bar which forms the furcula or "merrythought." In the Mammalia the Monotremata (Ornithorhynchus and Echidna) retain the reptilian arrangement of large coracoids and epicoracoids articulating with the sternum, while the clavicles and interclavicle are also largely developed; the scapula too is more bird-like in shape than mammalian. In the higher mammals
the scapula develops a spine and usually an acromial process, and has a triangular outline. As long as the forelimb is used for sup port, the vertebral border is the shortest of the three, and the long axis of the bone runs from this border to the glenoid cavity; but when the extremity is used for prehension, as in the Primates, or for flight, as in the Chiroptera, the vertebral border elongates and the distance from it to the glenoid cavity decreases so that the long axis is now parallel with that of the body instead of being transverse.
Above the monotremes too the coracoid becomes a mere knob for muscles, and no longer articulates with the sternum. There is thus a sudden transition from the way in which the forepart of the body is propped up on the forelimbs when the coracoid is func tional (as in reptiles) to the way in which it is suspended like a suspension bridge between the two scapulae in pronograde mam