BIRDS. 245 



or a " superior " and " inferior mandible." The upper man- 

 dible is composed almost entirely of the greatly-elongated 

 intermaxillary bones, flanked by the comparatively small 

 superior maxillas. The inferior mandible is primitively com- 



Fig. 581. — Skull of Spur-winged Goose {Plectropttrus Gavibensis). 



posed of twelve pieces, six on each side ; but in the adult 

 these are all indistinguishably amalgamated with one another, 

 and the lower jaw forms a single piece. As in the Eeptiles, 

 the lower jaw articulates with the skull, not directly, but 

 through the intervention of a distinct bone — the quadrate 

 bone or tympanic bone — which always remains permanently 

 movable, and is never anchylosed with the skull. In no 

 living bird are teeth ever developed in either jaw, but both 

 mandibles are encased in horn, forming the beak, and the 

 margins of the bill are sometimes serrated. In the Tertiary 

 Odontopteryx, however, the alveolar margins of the jaws are 

 prolonged into tooth-like processes sheathed in the horny 

 substance of the bill ; and in the Cretaceous Odontornitlics 

 true teeth are present. 



The thoracic cavity is bounded behind by the dorsal ver- 

 tebra, which are usually, as before said, anchylosed with one 

 another to a greater or less extent. Laterally, the thorax 

 is bounded by the ribs, which vary in number from six to 

 ten pairs. In most birds each rib carries a peculiar process 

 — the " uncinate process " — which arises from its posterior 

 margin, is directed upwards and backwards, and passes over 

 the rib next in succession behind, where it is bound down 

 by ligament. The first and last dorsal ribs carry no uncinate 

 processes, and in some cases the processes continue through- 



