208 



BIRD GALLERY. 



the Gannets and Screamers, these sacs also penetrate between the muscles 

 and beneath the skin. The bones which contain these air-sacs are 

 hollow or pneumatic and consequently have no marrow. In the Albatroses, 

 Gannets and Pelicans, which possess great powers of flight, almost every 

 bone in the body becomes pneumatic, but the Swifts and Swallows, which 

 possess equal powers, have the long bones filled with marrow. So also 

 have Penguins, Grebes, Divers, and the smaller Petrels. 



Skeleton [Plate XXIV. Figs. V.- VII.]. —As regards the details of the 

 structure of the Birds^ skeleton, we can only refer here to those points 

 Svhich are either generally characteristic of the Class or wliicb ai'c 

 strikingly correlated to the peculiarities of their life. The bones of the 

 cranium (PL XXIV. fig. V. 1) become united (coalesce) early in life, 



:, .' Fii?. VIII. 



Head of a Ffilooii (Hicrofalco islundui) to show (]) impervious nostrils, 

 and (2) tootli-like process of tlie bill. 



Fie-. IX. 



Head of tbe Tdack Turkey-Vulture (Catharistes uruhu) to show (1) perrious nostrils. 



about the period when growth ceases, so that the sutures between the 

 bones, which are persistent for so long a period in the Mammahan and 

 also in the Reptilian skull, disappear entirely. As in Reptiles, the skull 

 is joined with the neck by means of a single hinge or condyle (fig.VI. 1). 

 The orbits are of very large size in accordance with the great development 

 of the eye (fig. V. 4). The facial bones are more or less prolonged and 

 united to form the beak, which is covered with a horny sheath, the edges 

 of which may be notched (Barbets and Falcons) (fig. VIII. 2) or serrated 

 (Mergansers), but teeth are invariably absent in living forms. The 

 external nostrils are cither pervious (fig. IX. 1) or separated from one 



