PECTORAL GIRDLE — MAMMALS 



257 



process, the acromion, with which, when functional, the clavicle 

 articulates. Nothing certain is known of an episternum, except in 

 Monotremes (fig. 273). 



MoNOTREMES, in their early stages, have either half of the cora- 

 coscapular cartilage a continuum, with a marked incisure on the 



Fig. 272. — Pectoral girdle and sternum of Tatusia (Weber, '04). a, acromion; c, 

 clavicle; d, deltoid ridge; /«, entepicondylar foramen; h, humerus; if, infraspinous fossa; 

 p, presternum; sf, supraspinous fossa; sp, spina of scapula; x, xiphisternum. 



anterior border, possibly homologous with one of the lacertilian fenes- 

 trae. The cartilages of the two sides meet in the middle Hne. Three 

 bones ossify in each half — scapula, coracoid and precoracoid (epicora- 



^'^i^- 



Fig. 273. — Pectoral girdle of Ornithorhynchns. d, clavicle; co, coracoid; csf, 

 coraco-scapular fenestra; ec, precoracoid; es, episternum; g, glenoid fossa; 5, scapula; 

 si, sternum. 



coid of authors, see p. 229; figures 273, 274), the precoracoid ossifying 

 later than the others and retaining a cartilage (Pepicoracoid) on its 

 medial margin. Coracoid and scapula fuse early, but the precora- 

 coid is permanently separate and functional, the only case in mam- 



