XIII 



PHYLUM CHORDATA 



401 



Bird. When the mandible is 

 depressed the contraction of the 

 digastric muscle causes a forward 

 movement of the lower end of 

 the quadrate, which pushes for- 

 wards the maxillo - jugal bar 

 and the palatines and ptery- 

 goids, the latter sliding upon 

 the rostrum. Both the maxillae 

 a,nd the palatines are articulated 

 in front with the premaxilla and 

 together push it upwards ; in 

 this way depression of the lower, 

 produces an automatic raising of 

 the upper, jaw. The great size 

 and strength of both premaxilla 

 and mandible are remarkable, as 

 also is the fact that the orbit is 

 completely surrounded by bone, a 

 backward process of the lacrymal 

 being joined beneath it by a for- 

 ward process of the frontal. 



The mandible contains in the 

 young Bird the six bones on each 

 side characteristic of Reptiles ; 

 the coronary is, however, often 

 absent. As a rule the head of 

 the quadrate articulates with the 

 roof of the tympanic cavity by a 

 single facet in Ratitae, by a double 

 facet in Carinatae. The hyoid 

 always agrees in essential respects 



ode \ y* 



FIG. 1005 b is. Skull of Ara (Macaw). (From a photograph by 



A. Hamilton.) 



VOL. II 



FIG. 1005. Anas boschas (Duck). Ventral 

 view of Skull. a. p. f. anterior palatine 

 foramen ; b. o. basi-occipital ; b. pg. basi- 

 pterygoid process ; b. s. basi-sphenoid ; b. t. 

 basi-temporal ; e. o. ex-occipital ; eu. aper- 

 ture of Eustachian tube ; f. m. foramen mag- 

 num ; i. c. internal carotid foramen ; j. jugal ; 

 mx. maxilla ; mx. p. maxillo-palatine pro- 

 cess ; oc. c. occipital coudyle ; pi. palatine ; 

 p. n. posterior nares ; px. pre-maxilla ; q. 

 quadrate ; q. j. quadrato- jugal ; r. vomer : 

 IX, X, foramen for ninth and tenth nerves ; 

 XII, for twelfth nerve. (From Wieder- 

 sheim's Vertebrata.) 



with that of the Pigeon ; in the 

 Woodpecker the 

 posterior cornua 

 are curved round 

 the head and fixed 

 to the skull in the 

 neighbourhood of 

 the right nostril, a 

 very flexible and 

 protrusible tongue 

 being produced. 



The structure of 

 the shoulders-girdle 

 furnishes one of 

 the most funda- 

 mental distinctive 



D D 



