22 BIRDS. 



The nostrils are two in number, situated usually near the base of the maxilla. 

 Their form and exact position vary considerably in different birds. They often 

 open into a depression or groove (the nasal fossa or sulcus). In the Petrels 

 (Tubinares) the borders of the nostrils are prolonged forwards in a tubular form. 

 In some birds (as Hawks and Parrots) they open in a soft waxy-looking covering 

 of the base of the maxilla, called the cere. In the Apteryx alone of existing 

 birds the nostrils open near the tip of the bill. 



The horny sheath of the bill grows continuously from the base, but in con- 

 sequence of the wear to which the edges and the tip are subjected, it usually 

 maintains its definite size and form throughout life. If from any cause the 

 natural wear does not take place, the continuing growth produces a prolongation 

 of the upper mandible in either straight, curved, or spiral form (Nat. Hist. 

 Museum). 



TEETH OF BIRDS. 



The nearest approach to teeth in any existing birds is seen in certain modifica- 

 tions of the edges of the horny coverings of the bill. These may be — 



(i.) Dentate, having one or two tooth-like processes (Cerchneis tinnunculus, 

 Harpagus Iddentatus). 



(ii.) Serrate, having many such processes {Mergus serrator, Momotus hrasiliensis, 

 Pteroglossus baillom). 



(iii.) Lamellate, produced into numerous ridges or fine plates, which act rather 

 as strainers than as cutting or tearing organs (the Shoveller, Wild Duck, Goose, 

 Swan, Flamingo, etc.). 



In Odontopteryx toUapicus, a bird of the Eocene period, conical dry processes 

 were developed on the jaws, presumably covered with horn (as in the Tortoise, 

 Hardella tJiurgi), and closely resembling teeth functionally. 



As far as it is at present known, only in the Mesozoic epoch did birds exist 

 possessing true teeth composed of dentine, and with enamelled covered crowns. 

 These were implanted either in a continuous groove (as in Hesperornis regalis) 

 or in distinct sockets (as in Ichthyornis dispar and ArcJiceopteryx macrura). 



FORELIMB OR WING. 



The bones of the forelimb (wing) of a bird consist of humerus (upper arm), 

 radius and ulna (forearm), and manus (hand). Ulna usually stronger than 

 radius. There are only two carpal bones, one radial and one ulnar (excepting the 

 Screamer, C. chavaria, which has three). 



In Apterygidae and in Casuariidss there is but one complete digit in the manus, 

 and it is provided with a claw. In Struthionidse, Eheidse, and in all the Carinatse, 

 there are three digits in the manus, which answer to the pollex and second and third 

 digits of the pentadactyle forelimb, and the metacarpal bones of these digits are 

 ankylosed together. As a rule the metacarpal of the pollex is much shorter than 

 the other two ; that of the second digit is strong and straight ; that of the third 

 is more slender and bowed. 



The pollex has two phalanges, and the second of them is in many birds (Rhea, 

 Chauna, etc.) pointed, curved, and ensheathed in a horny claw. 



