406 ZOOLOGY SECT. 



recent orders, seems certain from the fact that the cretaceous Birds 

 were toothed. In Hesperornis (Fig. 993) there are long conical 

 teeth in both jaws, set in. a continuous groove. In Ichthyornis 

 (Fig. 994) the teeth are thecodont, like those in the Crocodile, 

 each being placed in a distinct socket. In Gastornis and in 

 Odontopteryx, an extinct carinate form allied to the Anseres, the 

 margins of the bony jaws are produced into strong, pointed, tooth- 

 like prominences. Vestigial teeth have been discovered in the 

 young of some Parrots. 



In the enteric canal the chief variations have to do with the size 

 of the crop and of the creca, in the gizzard, and in the coiling 

 of the intestine. In grain-eating Birds the gizzard has thick 

 muscular walls and is lined by a thickened horny epithelium, 

 as in the Pigeon : in flesh-eaters, such as Gulls, Petrels, Hawks, 

 and Owls, it is thin walled and lined with epithelium of the ordinary 

 character. It has been found by experiment that the carnivorous 

 gizzard of a Gull becomes thick- walled under the influence of a diet 

 of grain while the converse change is produced by feeding a 

 Pigeon with meat. In the Common Fowl and many other Birds 

 the cceca are of great length. A gall-bladder is usually present : 

 the spleen is always small. The tongue may be pointed, as in the 

 Pigeon ; very long and protrusible, as in Woodpeckers ; short and 

 thick, as in Parrots; or modified for honey-sucking by the tip 

 being produced either into a brush-like organ or into paired 

 sucking-tubes. There are variously situated ~buccal glands, to some 

 of which the name salivary is often applied. 



Respiratory and Vocal Organs. The rings of the trachea 

 are always ossified : the tube is frequently deflected to one 

 side by the crop, as in the Pigeon, and may undergo such 

 an increase in length as to extend beneath the skin of the 

 abdomen, or even into the keel of the sternum. The syrinx is 

 either tracheo-bronchial, as in the Pigeon, i.e., formed by the distal 

 end of the trachea and the proximal ends of the bronchi, or is 

 exclusively tracheal or exclusively bronchial. In singing Birds it 

 is complex, and is provided with numerous muscles five or six 

 pairs for altering the tension of the vibrating membrane. 



The lungs are always firmly fixed to the dorsal body- wall by a 

 pulmonary aponeurosis, and are but slightly distensible. The 



General arrangement of the air sacs has been described in the 

 igepn (p. 371) : in Apteryx the abdominal air sacs are small, and 

 are completely enclosed by the oblique septum, so as not to extend 

 into the abdominal cavity among the viscera. The bronchi send 

 off branches at right angles. 



The Circulatory Organs agree in all essential respects with 

 those of the Pigeon : their most characteristic features are the large 

 size of the heart, the muscular right auriculo- ventricular valve, 

 the atrophy of the left aortic arch, and the vestigial character 



