The ossification of the shoulder-girdle has gone on much further in Anura than it has in Urodela. Clavicles are present and the scapula and coracoid of each side are ossified from separate centres. The distal part of the scapula forms a large imperfectly ossified plate, the suprascapula.
The shoulder-girdle of Anura is however subject to considerable variations. In the Toads (Bufonidae) the epicoracoids or unossified ventral ends of the coracoids and precoracoids overlap in the middle line (fig. 30, B, 5). This arrangement is called Arciferous. In the Frogs,—Ranidae, and other forms belonging to the group Firmisternia,—the epicoracoids do not overlap but form a narrow cartilaginous bar separating the ventral ends of the coracoids (fig. 30, A, 5).
Anterior limb.
In many Amphibia and especially in the Urodela the anterior limb has a very simple unmodified arrangement. The humerus is straight and of moderate length, its ends are rounded for articulation on the one hand with the shoulder-girdle, and on the other hand with the radius and ulna. In the Urodela the radius and ulna are distinct. In the Anura they have fused, though the line of junction of the two is not obliterated. Their proximal ends are hollowed for articulation with the convex end of the humerus.
The manus in all recent Amphibia agrees in never having more than four complete digits, but is subject to considerable variation, this statement applying especially to the carpus.
In the larva of Salamandra (fig. 31, A), except that the pollex is absent[64], the manus retains completely the condition which is generally regarded as primitive for the higher Vertebrata. It consists of an anterior row of three elements, the ulnare, intermedium, and radiale, and a posterior row of four, the carpalia 2, 3, 4, and 5. Interposed between the two rows is a centrale. Menobranchus has a similar very simple carpus. In most other Amphibia this simplicity is lost. This loss may be due to:—
(a) fusion of certain structures, e.g. in the adult Salamandra the intermedium and ulnare have fused,
(b) displacement of structures, e.g. in Bufo viridis, the centrale has been pushed up till it comes to articulate with the radius,
(c) the development of supernumerary elements, especially of extra centralia. In Megalobatrachus two or even three centralia sometimes occur.
