256 VERTEBRATE SKELETON 



figure 271,^, Dronmus, Apteryx) the fenestra is a gap and the precora- 

 coid is a process extending forwards and inwards from the glenoid 

 region. The same process, in various stages of reduction, occurs in 

 many Carinates. The Carinates are more primitive than Ratites 

 in that the scapula and pillar-like coracoid meet at an angle, the 

 coracoid extending to the sternum, thus acting as a brace against 

 the action of the pectoral muscles. Carinates also have a consider- 

 able acrocoracoid process (scarcely recognizable in Ratites) beginning 

 •as a crest on the ventral side of the coracoid and extending from the 

 glenoid region to support the broad upper end of the clavicle. 



Most Carinates have the clavicles at some distance from the cora- 

 ccids, except at the upper ends. These bones are long and slender, 

 their strength and curvature parallel with the powers of flight, the 

 extreme occurring in the diurnal Raptores. The two clavicula are 

 fused at their ventral ends, forming the furcula ('wishbone'), the 

 place of fusion having a median discoid process in Oscines and 

 Gallinae. The upper ends of the furcula articulate with the acrocora- 

 coid process and (a few Carinates excepted) with the acromion of 

 the scapula. In all Ratites (fig. 271,^) and some Carinates (pigeons, 

 humming birds) the clavicles are more or less reduced, so far in some 

 parrots and DromcBus that only a small rudiment rests against the 

 acrocoracoid, while in other parrots and Ratites it is lost. Even 

 when better developed, the fused ends do not usually meet the ster- 

 num, but are connected with it by hgament (fused with the carina 

 in Steganopods). 



It is stated that part of the clavicle is laid down in cartilage, but this needs 

 confirmation. There is also said to be a.cartilage at the junction of the two halves 

 of the furcula, regarded by some as traces of an episternum, otherwise absent 

 from birds, but the cartilage renders such homology doubtful. 



MAMMALIA show varying degrees of mobihty between the 

 bones of the pectoral girdle, which, as a whole, presents two different 

 conditions, the more primitive in Monotremes (figure 273, where 

 relations are much as in Lacertilia and especially in Therapsida), 

 the other in PlacentaHa; fetal Marsupials (fig. 275) are intermediate, 

 although the adults are more like Placentals. In all the scapula is 

 broad above, narrow at the glenoid end, and bears a separate supra- 

 scapula only in Monotremes and some Ungulates. It has a strong 

 ridge (spina) on the outer surface for greater muscular attachment, 

 dividing the surface into two fossic, and terminating in a ventral 



