In some larval Labyrinthodontia (_Branchiosaurus_) four branchial arches are known to occur, and their arrangement is almost precisely similar to that in _Siredon_. In Gymnophiona the remains of only three branchial arches occur in addition to the hyoid. The four arches are all very similar to one another, each consists of a curved rod of uniform diameter throughout. The hyoid is united with the first branchial arch, but has no attachment to the cranium. In larval Anura (fig. 29, C) the arrangement of the hyoid and branchial arches is much as in Urodela. In the adult, however, the ventral parts of all the arches unite, forming a compact structure, the _basilingual plate_ (fig. 29, B, 1). [Illustration FIG. 29. VISCERAL ARCHES OF AMPHIBIA. A. _Molge cristata_ (after PARKER). B. _Rana temporaria_ adult (after PARKER). C. Tadpole of _Rana_ (after MARTIN ST ANGE). D. _Siredon pisciformis_ (after CREDNER). In each case the ossified portions are slightly shaded, while the cartilaginous portions are left white. 1. basilingual plate. 2. hyoid arch. 3. first branchial arch. 4. second do. 5. third branchial arch. 6. fourth do. 7. thyro-hyal. 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.