[Illustration FIG. 30. SHOULDER-GIRDLE AND STERNUM OF
A. An old male common Frog (_Rana temporaria_). B. An adult female _Docidophryne gigantea_ (after PARKER).
In both A and B the left suprascapula is removed. The parts left unshaded are ossified; those marked with small dots consist of hyaline cartilage, those marked with large dots of calcified cartilage.
1. calcified cartilage of suprascapula. 2. ossified portion of suprascapula. 3. scapula. 4. coracoid. 5. epicoracoid. 6. precoracoid. 7. clavicle. 8. glenoid cavity. 9. coracoid foramen. 10. episternum. 11. omosternum. 12. sternum. 13. xiphisternum.]
In most Anura the sternum consists of a number of parts arranged in series. At the anterior end is a flat cartilaginous plate with a bony basal stalk. This plate is called the episternum, and its stalk the omosternum. The continuity of the sternum is now interrupted by a pair of cartilaginous structures, the epicoracoids, which are shoulder-girdle elements, and represent the unossified ventral ends of the coracoids. In some cases cartilaginous epiprecoracoids can also be distinguished. Further back is the long sternum proper, while last comes the xiphisternum, a broad expanded plate of cartilage.
In some Anura such as _Pipa_ and _Hyla_ the number of sternal elements is considerably reduced.
The most primitive Amphibian shoulder-girdle is found in the Urodela. It consists of a dorsal element, the scapula, a posterior ventral element, the coracoid, and an anterior ventral element, the precoracoid. The clavicle is not developed, and the two coracoids overlap in the middle line. The shoulder-girdle remains largely cartilaginous but the proximal end of the scapula is ossified, and the ossification may extend through part of the coracoid and precoracoid.
In Labyrinthodontia there is an exoskeletal ventral buckler formed of three plates, a median one, which probably represents an interclavicle, and two lateral ones, which are probably clavicles. Traces of endoskeletal structures, probably corresponding to the precoracoid and scapula, are also known in some cases. The Gymnophiona and some of the Labyrinthodontia have lost the pectoral girdle and limbs.
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.