258 



VERTEBRATE SKELETON 



mals. The curved scapula of the adult is relatively narrow, has a 

 short spine or none, and bears a short acromion bent ventrally on its 

 anterior margin. The clavicles of the two sides meet in the young; 

 in the adult their median ends are separate and each is closely 

 appressed to, and in adults fused with the transverse arms of the 

 episternum, which, in turn, fuses with the more dorsal sternum. 

 Marsupialia. — In embryo Marsupials (fig. 275)' the coracoid 

 part of the cartilage girdle abuts on the presternum, but is relatively 

 much shorter than in Monotremes. This connexion is lost in the 

 adult and the whole is much as in PlacentaUa, the coracoid being 



Fig. 274. — Dorsal side of sternum and 

 pectoral girdle of Echidna (Parker, '68). 

 c, coracoid; cl, clavicle; es, episternum; 

 p, presternum; 5, sternebrae. 



Fig. 275. — Pectoral girdle and sternum 

 of Dasyurus from pouch (Broom, '02). 

 a, acromion; c, clavicle in undifferentiated 

 tissue; co, coracoid; p, presternum; s, 

 scapula; sp, spina; st, sternum. 



small and somewhat hooked. The clavicle (none in bandicoots) 

 extends from acromion to sternum, 'omosternal' cartilages inter- 

 vening. It is not always completely ossified. No episternum occurs. 

 PLACENTALIA have the ventral parts of the girdle greatly 

 reduced, reduction sometimes extending to complete loss of the clavi- 

 cle, while the coracoidal parts are a small coracoid process fused 

 usually with the lower end of the scapula. The scapula is well 

 developed, is usually triangular (sometimes quadrangular) in 

 outHne, and is plate-like, with a strong spine terminating in an acro- 

 mion except in whales. The acromion bends ventrally, often extend- 

 ing beyond the shallow glenoid fossa, and articulates with the clavicle 

 when the latter is present. The spina is close to the anterior margin 

 in lower mammals, in the higher the prespinous part is increased so 



