208 



1)11(1) GALLKltV. 



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

 ami beneath the skin. The bones whicli contain these air-sacs are 

 liollow or i)neumatic and consequently have no marrow. In the Albatrose.s, 

 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 tlie details of the 

 structure of the Birds' skeleton, wc can only refer here to those points 

 which are either generally characteristic of the Class or which are 

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

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



Fi-. VIII. 



Head ol'.n l'"alcoii (Hiififalco i.<hin<Iiix) to show (1) iiupoivious nostrils, 

 .inil ('2) tooth-like process of the bill. 



Fi- IX. 



Head ol'thc IMaelc '^ul■liey-^'llltul•e (Vnthuriste^ iinibu) to show (1 ) pervious nostrils. 



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

 tiones, which are persistent for so long a period in the Mammalian 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.A^I. 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 tlie beak, wliich is covered with a horny sheath, tlic 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 either pervious (fig. IX. 1) or separated from one 



