184 THE VERTEBRATE SKELETON. 



arranged in series. At the anterior end is a flat cartilaginous 

 plate with a bony basal stalk. This plate is called the epi- 

 stermim, 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 distin- 

 guished. 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. 



APPENDICULAR SKELETON. 

 PECTORAL GIRDLE. 



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 de- 

 veloped, 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 repre- 

 sents an interclavicle, and two lateral ones, which are probably 

 clavicles. Traces of endoskeletal structures, probably corre- 

 sponding to the precoracoid and scapula, are also known in 

 some cases. The Gymnophiona and some of the Labyrintho- 

 dontia 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. 



