336 THE VERTEBRATE SKELETON. 



The hyoid apparatus (fig. 59, C) consists of a median 

 portion, and a pair of cornua. The median portion is com- 

 posed of three pieces placed end to end, and called respectively 

 the os entoglossum, the basi-hyal, and the uro-hyal. The os 

 entoglossum is shown by development to be formed by the 

 union of paired structures and is probably homologous with the 

 hyoid arch of fishes. The basihyal and the long cornua, each 

 of which is composed of two or three pieces placed end to end, 

 are homologous with the first branchial arch of fishes, while 

 the urohyal is probably homologous with the second branchial 

 arch of fishes. In Woodpeckers the cornua are enormously 

 long, and curve over the skull, extending as far forwards as 

 the anterior nares. 



RIBS AND STERNUM. 



Well-developed ribs are attached to the posterior cervical 

 vertebrae as well as to the thoracic vertebrae. The ribs 

 generally have uncinate processes and separate capitula and 

 tubercula, but uncinate processes are absent in Chauna Pala- 

 medea and apparently in Archaeopteryx. 



The sternum (fig. 63) is greatly developed in all birds. In 

 the embryo 1 it is seen to be derived from the union of right 

 and left plates of cartilage, formed by the fusion of the ventral 

 ends of the ribs. In the Ratitae and a few Carinatae, such 

 as Stringops, it is flat, but in the great majority of birds 

 it is keeled, though the development of the keel varies greatly. 

 It is large in the flightless Penguins, which use their wings 

 for swimming. Traces of an interclavicle may occur in the 

 embryo. 



PECTORAL GIRDLE. 



The pectoral girdle is also strongly developed in all 



Carinatae, but is much reduced in Ratitae. In some Moas 



the sternum has no facet for the articulation of the coracoid, 



and the pectoral girdle appears to have been entirely absent ; 



1 B. Lindsay, P. Z. S. 1885, p. 684. 



