?46 



VERTEBRATE SKELETON 



While it is possible to recognize broad homologies between piscine 

 and Tetrapod pectoral girdles, there are difficulties in details and 

 several of the statements below are only tentative. There is a 

 striking degeneration and loss of parts in the higher groups — replace- 

 ment of precoracoid by clavicle, disappearance of epicoracoid and 

 episternum, reduction of coracoid, and in many mammals loss of 

 clavicle. 



AMPHIBIA.- — Girdles are absent from Aistopod Stegocephals 

 and Gymnophiona, while the diversity in other orders is greater than 



in some other classes. The prob- 

 ably neotenic character of the 

 Urodeles results in an almost 

 embrycnic condition of the girdle, 

 membrane bones being absent, 

 although probably present in the 

 ancestors. 



Fig. 259.-Pectoral girdle of Eryops StEGOCEPHALA (Aistopoda CX- 



(Gregory, '15). c, coracoid; ct, cleithrum; ceptcd) haVC boneS in the thoracic 

 cv, clavicle; e, episternum; sc, scapula. . _ ._ >. . 



region. In some (iig. 259) there is 

 a continuous coraco-scapula without indications of division; in front 

 of this is a curved bone (clavicle or cleithrum), the broader medial 

 end of which is ventral to the broad episternum. In other genera 

 the identification of the separate bones (some apparently centers 

 of ossification in larger cartilages) is not certain. The episternum 



Fig. 260.- — Pectoral girdles of (.4) Necturiis, (B) Spelerpes, and C, Salamandrina 

 (Wiedersheim, "75). c, coracoid; g, glenoid fossa; p, precoracoid; .j, scapula; 55, supra- 

 scapula. Cartilage stippled, bone lined. 



(usually a rhomboid plate) is in the median line, with the broader 

 ends of a pair of clavicles (? cleithra) ventral to it. Behind these is 

 a pair of slender spHnts, interpreted as scapulae, followed by a pair 

 of larger, often semicircular, bones, possibly coracoids. 



