In the Urodela

In the Urodela the scapula ossifies, and its ossification may be prolonged into the coracoid and precoracoid, but there is never more than one osseous mass. The clavicle is not developed. In Siredon, the Derotremata, and Salamandridea, the coracoids are received into grooves of the anterolateral edges of a cartilaginous sternum.

The pectoral arch of the Labyrinthodonts seems to have possessed representatives of clavicles in the lateral thoracic shields. The structure of the rest of the arch is not clear, but ossified coraco-scapular pieces seem to have existed.

In the Batrachia, the coraco-scapular cartilages are sometimes, as in the common Frog, firmly united in the middle line, and send forward a median process, which becomes ossified, and is the omosternum (Fig. 57, o.st.). Behind the coracoids articulate with a well-developed sternum (st.).

Distinct ossifications arising on either side of the glenoidal cavity represent the scapula (sc.) and the coracoid (cr.), and the upper moiety of the scapula may be distinctly ossified as a suprascapula (s.sc). The coracoid is divided by a large membranous space ov fontanelle into a proper coracoid (cr.), which lies behind the fontanelle; a persistently cartilaginous epicoracoid (e.cr.), which bounds it internally; and a precoracoid, which limits it in front. Closely applied to the precoracoid is an ossification in membrane, which represents the clavicle.

The pelvic arch is attached (except in Proteus) to the extremity of the sacral rib. An iliac ossification is always developed; an ischial, in all but Proteus. The pubis does not appear to be regularly represented by a distinct ossification. In the Batrachia the applied flat faces of the expanded ventral divisions of the pelvic arch coalesce into a disk.

In the genus Amphiuma, the limbs have each either two or three digits. In Siren, the anterior limbs, which alone exist, are three-or four-toed. In Proteus, the anterior limbs are tridactyle, the posterior didactyle. Menobranchus has tetradactyle feet, while in the other Urodela the anterior limbs are tetradactyle, the posterior pentadactyle. The Batrachia have four digits, with or without a rudiment of another, in the fore-limb, and five in the hind-limb. In the perennibranchiate Urodela, the cartilages of the carpus and tarsus, which, except in Proteus, present little deviation from the typical number and arrangement (Fig. 11, p. 32), remain unossified; in the other Urodela, and in the Batrachia, they are for the most part ossified.

In the Urodela the tail persists, and develops complete vertebrae; but, in the Batrachia, the caudal part of the spinal column disappears, for the most part, together with the rest of the tail, and only the basal portion of the notochord becomes converted into the urostyle, which eventually anchyloses with the two hindermost neural arches.