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. [Illustration FIG. 31. A, RIGHT ANTIBRACHIUM AND MANUS OF A LARVAL SALAMANDER (_Salamandra maculosa_) (after GEGENBAUR). B, RIGHT TARSUS AND ADJOINING BONES OF _Molge sp._ (after GEGENBAUR). 1. radius. 2. ulna. 3. radiale. 4. intermedium. 5. ulnare. 6. centrale. 7. carpale 2. 8. " 3. 9. " 4. 10. " 5. 11. tibia. 12. fibula. 13. tibiale. 14. intermedium. 15. fibulare. 16. centrale. 17. tarsale 1. 18. tarsalia 4 and 5 fused. I. II. III. IV. V. digits.] In the great majority of Amphibia while one digit, probably the first, is absent, the other four digits are well developed. In the forms however with degenerate limbs like _Amphiuma_, _Siren_ and _Proteus_ the number of digits is still further reduced. In _Siren_ there are three or four, in _Proteus_ three, and in _Amphiuma_ two or three digits in the manus.