180 THE ANATOMY OF VEETEBEATED ANIMALS. 



thodonts the articular element of the lower jaw is com- 

 pletely ossified. 



Arcliegosaurus possessed branchial arches when yoimg, 

 and there can be little doubt that the other Labyrintho- 

 donts resembled it in this respect. 



The limbs and their arches are completely absent in the 

 Gymnophiona, and, apparently, in the extinct Opliiclerpeton 

 of the Carboniferous formation. In all other Amphibia the 

 pectoral arch and limbs are present, and, in aU but Siren, 

 the pelvic arch and limbs. The anterior and posterior 

 limb arches consist of a continuous cartilage on each side, 

 divided by an articular surface into a smaller dorsal moiety, 

 and a more expanded ventral portion. The dorsal moieties 

 are, respectively, the scapula and the ilium. The ventral 

 moieties are divided by notches, or fontaneUes, into two 

 portions — an anterior, precoracoidal, or pubic part, and a 

 posterior, coracoidal, or ischial part. 



In the Urodela the scapula ossifies, and its ossification 

 may be prolonged into the coracoid and precoracoid, but 

 there is never more than one osseous mass. The clavicle 

 is not developed. In Siredon, the Derotremaia, and 8ala- 

 mandridea, the coracoids are received into grooves of the 

 anterolateral edges of a cartilaginous sternum. 



The pectoral arch of the Labyrinthodonts seems to have 

 possessed representatives of clavicles in the lateral thoracic 

 shields. The structure of the rest of the arch is not clear, 

 but ossified coraco-scapular pieces seem to have existed. 



In the Batrachia, the coraco-scapular cartilages are some- 

 times, as in the common Frog, firmly united in the middle 

 line, and send forwards a median process, which becomes 

 ossified, and is the omosternum (Fig. 57, o.st.). Behind, 

 the coracoids articulate with a weU-developed sternum {st.). 

 Distinct ossifications arising on either side of the glenoidal 

 cavity represent the scapula (sc.) and the coracoid (cr.), and 

 the upper moiety of the scapula may be distinctly ossi- 

 fied as a supra-scapula (s.sc). The coracoid is divided by a 

 large membranous space or fontanelle into a proper cora- 

 coid (cr.), which lies behind the fontanelle ; a persistently 



