PECTORAL GIRDLE ^BIRDS 



255 



Pterosauria usually have coracoid and scapula fused (fig. 270), the scapular 

 part being long and slender and sometimes (Pteranodon) attached to the anterior 

 vertebrae. The coracoid is slender and articulated by a synovial joint with the 

 sternum. Clavicles are absent. 



AVES. — Making allowances for modifications connected with 

 flight, there is not much difficulty in comparing the pectoral girdle of 

 birds with those of lizards, although some homologies are not clear. 

 The adult girdle consists of three distinct bones on either side — ■ 

 scapula, coracoid and clavicle — the precoracoid being reduced and 

 no episternum present. The glenoid fossa is formed entirely by 

 coracoid and scapula. 



The scapula, which lacks a spine, is long and slender (its dorsal 

 end expanded in penguins) and is placed differently in Ratites 



Fig. 271. — Pectoral girdles and sterna of birds. A, Casuarius (Parker, '68); B, C, 

 young Struthio catnelus (Parker) ; D, adult S. australis; E, Haliatus leiicoce phala (Shu- 

 feldt, '09). c, coracoid; cf, coracoid fenestra; cl, clavicle; e, epicoracoid; g, glenoid 

 fossa; p, precoracoid; so, sternal ossification; s, scapula; cartilage stippled. 



and Carinates. In both it is dorsal to the ribs, running back parallel 

 to the vertebrae in Carinates, ascending upwards and backwards in 

 Ratites, where the distal end is near the spinal column. It usually 

 has a considerable acromial process on the inner ventral end which 

 articulates with the upper end of the clavicle, the coracoid connecting 

 with its external side. It is the most frequently pneumatic of any 

 of the girdle bones, the opening being near the proximal end. 



Although the coracoid is reduced in Ratites and fused in a straight 

 line with the scapula in Struthio (fig. 271, C, Z)), it is primitive in its 

 width and in having a gap which partially separates an anterior bar 

 (fig. 271, C), the precoracoid. In other birds (Raptores, figure 271, £) 

 the fenestra is reduced to a foramen while in other Ratites {Casuarius, 



