In each case the ossified portions are slightly shaded, while the cartilaginous portions are left white.

1. basilingual plate.5. third branchial arch.
2. hyoid arch.6. fourth do.
3. first branchial arch.7. thyro-hyal.
4. second do.8. copula.

The dorsal parts of the first three branchial arches disappear, but those of the fourth become ossified and form the short, stout thyro-hyals or posterior cornua. The dorsal parts of the hyoid arch in the adult form a pair of long bars, the anterior cornua, which are united to the periotic region of the skull in front of the fenestra ovalis either by short ligaments or by fusion as in Bufo. In Pipa and Xenopus the first and second branchial arches persist as well as the fourth (thyro-hyal), but in Pipa the hyoid is wanting.

Ribs.

Ribs are generally very poorly developed in Amphibia. In Anura they are in most cases absent; when present they generally form minute unossified appendages attached to the transverse processes, but in Discoglossus and Xenopus the anterior vertebrae are provided with distinct ribs. In Urodela and Labyrinthodontia they are generally short structures, each as a rule attached to the vertebra by a bifurcated proximal end. The number of rib-bearing vertebrae varies, but the first and the posterior caudal vertebrae are always ribless. The anterior caudal vertebrae too are generally ribless, but sometimes a few of them bear small ribs. In Spelerpes the last two trunk vertebrae are ribless, and hence may be regarded as lumbar vertebrae.

In Gymnophiona ribs are better developed than in any other Amphibia; they occur on all the vertebrae except the first and last few, and are attached to the transverse processes, sometimes by single, sometimes by double heads.

Sternal ribs are almost unknown in Amphibia, but traces of them occur in Menobranchus.

Sternum.

In Amphibia the sternum is not very well developed; sometimes as in Gymnophiona and Proteus no traces of it occur, and in the Urodela it is never ossified. It is always very intimately related to the pectoral girdle. In the Salamandrina it has the form of a broad thin plate of cartilage, grooved and overlapped by the coracoid.