310 AMPHIBIA xii. 8 



dermal girdle was attached to the post-temporal region of the skull, 

 as in bony fish. This connexion was soon lost in later forms, pre- 

 sumably to permit greater mobility of the head. This foreshadowed 

 the reduction and loss that was the subsequent fate of the dermal, 

 shoulder girdle elements in tetrapod evolution. 



Of the modern amphibia, the Salientia most nearly approach the 

 condition of the fossil forms and they, alone, of recent tetrapods, have 

 retained a cleithrum. Each half of the endochondral girdle consists 

 of a dorsal, bony scapula with a cartilaginous suprascapula, and a 

 ventral coracoid bone connected to an anterior precoracoid cartilage 

 by a mesial epicoracoid cartilage. The precoracoids are invested by 

 the clavicles and, as in all modern amphibians, the interclavicle is 

 absent. 



Anuran shoulder girdles may be divided into two broad categories 

 according to whether the two epicoracoid cartilages are fused mesi- 

 ally (a) along their entire lengths (firmisternal condition) or (b) along 

 their anterior edges only (arciferal girdles). The latter occurs typically 

 in 'walking', toad-like Anura (e.g. Bufonidae, Pelobatidae) and in the 

 aquatic xenopids. The clavicles are the main struts for keeping the 

 glenoids apart and, consequently, they are well developed and never 

 lost. The coracoids, on the other hand, may only be moderately well 

 developed. Immediately behind their point of fusion the epicoracoid 

 cartilages diverge and overlap and their posterior margins are con- 

 tinued as epicoracoid horns, which run in lateral grooves on each side 

 of the sternum. The posterior tip of each horn has a muscle attach- 

 ment connecting with abdominal recti. This type of sternum/epi- 

 coracoid system permits a certain degree of independent movement of 

 the girdle halves whilst, at the same time, preventing the epicoracoid 

 cartilages from being forced too far apart. The mechanism clearly 

 facilitates the independent arm movements characteristic of locomo- 

 tion in the arciferal frogs. 



The firmisternal girdle is a rigid structure allowing no independent 

 movement of the two halves (Fig. 183). It occurs typically in frogs 

 with a jumping habit (e.g. Ranidae, Microhylidae) and provides an 

 excellent landing mechanism. The glenoids are braced apart by the 

 large coracoids. The clavicles and precoracoids are thus deprived of 

 their strutting function and frequently become reduced or even com- 

 pletely lost. No epicoracoid horns are present and the sternum, no 

 longer involved in locking the girdle halves, serves principally for the 

 attachment of pectoral muscles. This function is also performed, in 

 some frogs, by a prezonal (omosternal) element, which is really an 



