434 ZOOLOGY 



SECT. 



of the recent orders, seems certain from the fact that the 

 Cretaceous Birds were toothed. In Hesperornis (Fig. 1053) there 

 are long conical teeth in both jaws, set in a continuous groove. 

 In Ichthyornis (Fig. 1054) the teeth are theeodont, like those 

 of 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. Supposed 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 caeca, with the gizzard, and with 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. In the Common Fowl and many other Birds the casca 

 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 arid 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-lronchial, 

 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 ex- 

 clusively 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 

 Pigeon (p. 400) : 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 aorlic arch, and the vestigial character 

 of the renal portal system. The red blood-corpuscles are always 

 oval and nucleated. 



Nervous System and Sense-Organs. The brain is also 



