296 THE VERTEBRATE SKELETON. 



to any backwardly-projecting process or processes. The sternum 

 ossifies from two or more centres. 



The skull is extremely light, and its component bones show 

 a great tendency to fuse together completely. The facial part 

 of the skull is prolonged into a beak, chiefly formed of the pre- 

 maxillae; this beak is in all modern birds devoid of teeth, 

 and is coated externally with a horny epidermal sheath. The 

 quadrate is large and freely movable. The supratemporal 

 arcade 1 is imperfect, while the infratemporal arcade 1 is com- 

 plete. There are no postorbital or postfrontal bones. Neither 

 parotic processes nor an interparietal foramen occur. There 

 are commonly large pre-orbital vacuities. The palatines 

 and pterygoids never form a secondary bony palate as in 

 Crocodiles. Part of the floor of the skull is formed by a 

 wide basitemporal (paired in the embryo) which is continued 

 in front as a long slender rostrum ; these structures have re- 

 placed the parasphenoid of Ichthyopsids. Cartilage or bone is 

 always developed in the sclerotic. The first branchial arch is 

 well developed, the hyoid arch but slightly. The coracoids 

 are large, and the clavicles are nearly always united forming 

 ihefurc^da. There is no separate interclavicle and hardly any 

 trace of a precoracoid. 



The anterior limbs form wings, and the manus is in the 

 adult always much modified, never having more than three 

 digits. The three bones of the pelvis are, except in Archaeor- 

 nithes, always ankylosed together in the adult, and the ilium 

 is greatly prolonged in front of the acetabulum, which is 

 perforated. The ilia are not connected with the sacrum by 

 ossified sacral ribs. The pubes arid ischia are directed back- 

 wards parallel to one another, and except in a very few forms 

 never meet their fellows in ventral symphyses. The fibula 

 is generally much reduced. The proximal tarsal bones are 

 always ankylosed to the tibia, and the distal tarsals to the 

 metatarsals, so that the ankle joint is intertarsal. The first 

 1 See p. 283. 



