Il6 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



cleithrum. A large round element is called the coracoid. In the 

 recent amphibians we are on firmer ground. The halves of the girdle 

 develop separately, and the cleithrum is lacking. In urodeles the 

 coracoid region has two processes diverging from the glenoid fossa, 

 an anteriorly directed procoracoid and a coracoid proper, directed 

 toward its fellow of the opposite side, the two meeting the sternum 

 behind and overlapping in front. Ossification begins in the neigh- 

 borhood of the glenoid fossa, the resulting bone being called the 

 scapula, although it invades the coracoid region, the cartilage dorsal 

 to it being the suprascapula. 



In the toads and allied anura (arcifera) the halves of the girdle 

 overlap as in the urodeles, but the procoracoids extend toward the 

 middle line, each being joined to its coracoid by longitudinal cartilage 

 plate, the epicoracoid, leaving a gap between them. With the 

 appearance of bone, scapula and coracoid ossify, while a clavicle 



Fig. 122. — Arciferous girdle of Ceralophrys ornalus. d, clavicle; co, coracoid; e, 

 epicoracoid; h, head of humerus; s, scapula; ss, suprascapula; cartilage dotted. 



of membranous origin overlies the procoracoid cartilage (fig. 122). 

 In the frogs (firmisternia) the relations are much the same (fig. 63), 

 except that the epicoracoids, instead of overlapping, abut against 

 each other, and the clavicles nearly or quite replace the procoracoid, 

 while sternum and omosternum join the girdle in front and behind. 

 Girdles are lacking in the gymnophiones. 



REPTILES. — With the development of a considerable neck in 

 the reptiles the pectoral girdle is removed further from the head; 

 it shows considerable differences in the various groups. In the fossil 

 rhynchocephals it is much as in the stegocephals, except that the 

 scapula is large. In the turtles it occupies a peculiar position, being 

 inside the carapace, i.e., internal to the ribs; but this is explained by 

 the development; the girdle arises in front of the ribs and later sinks 

 to the definitive position. Scapula, procoracoid and coracoid are 

 well developed, the medial ends of the latter two being connected 

 by a cartilaginous epicoracoid. Elsewhere in the reptiles the pro- 

 coracoid tends to reduction, the clavicle taking its place, though it 



