SKULL. 429 



premaxillae (Jmx) are united into a large triradiate bone. The upper 

 beak is in some birds slightly moveable upon the skull at the posterior ends 

 of the nasals and premaxillae, and in the parrots there is a joint at this 

 point in virtue of which the upper beak has considerable mobility. 



There are two vomers but they early unite into a single bone, which is 

 sometimes extremely small or even absent. They underlie the median 

 ethmoid and, except in the ostrich, unite posteriorly with the palatines. 

 The maxillae, which are slender, possess maxillo-palatine plates which 

 may or may not unite with the vomer or with each other ventral to the 

 vomer. The palatines (Pal) are elongated bones extending from the pre- 

 maxillae backwards to the pterygoids, passing ventral to the maxillo- 

 palatines ; their hind ends usually join the basisphenoidal rostrum (Spb) 

 by an articular surface which allows of their movement on the rostrum. 

 The ptergyoids (Pt) in front articulate with the palatines and usually with 

 the basisphenoidal rostrum ; in some birds there is an additional articu- 

 lation with the rostrum by means of the basipterygoid processes of the 

 latter (Ratitae, some Carinatae). Posteriorly the pterygoids articulate with 

 a process of the quadrate. In struthious birds the pterygoids articulate 

 in front with the vomer (except in the ostrich). This also happens in the 

 embryos of many birds, in which the pterygoid extends forwards to the 

 vomer dorsally to the palatine. This forward process in later growth 

 loses its connection with the pterygoid, appearing to segment off from it ; 

 it sometimes remains as a separate splint called the hemipterygoid (pen- 

 guins, etc.), but it usually disappears, fusing indistinguishably with the 

 palatine. In Tinamus the pterygoid articulates with the vomer in the 

 adult as in struthious birds. 



The principal forajnina are as follows : the condylar foramen for the 

 12th nerve, through the exoccipital bone slightly in front of and ventral 

 to the foramen magnum. Slightly external to and behind the condylar 

 foramen is the jugular foramen for the 9th, 10th, and llth nerves and for 

 the internal jugular vein, between the periotic (petrosal) and the exocci- 

 pital bones. To the outer side of the jugular foramen is a depression, the 

 tympanic recess, at the median anterior end of which is the opening into 

 the carotid canal for the internal carotid artery. The 7th nerve traverses 

 the periotic bone and emerges by a small foramen in front of the fenestra 

 ovalis. The eustachian canals open into a deep notch at the anterior end 

 of the basitemporal, and just external to these are the anterior openings 

 of the carotid canals. The foramen for the trigeminal is just in front of 

 the articulation of the quadrate and is between the prootic and alisphenoid. 

 The optic foramen is a large foramen in the hinder end of the interorbital 

 septum ; and just behind it are two or three small openings for the oph- 

 thalmic branch of the 5th, the 3rd, 4th, and 6th nerves. The olfactory 

 foramen is dorsal to the optic and is continued as a groove between the 

 interorbital septum and the frontal. 



Huxley * pointed out the following different arrangements in the palatal 

 bones of birds : In the Ratitae and Tinamus the vomer is large and 

 broad, and the palatines do not articulate with the rostrum, the vomer 

 intervening ; there are well marked basipterygoid processes, which arti- 

 culate with the hinder parts of the pterygoid ; the maxillo-palatines unite 

 with the vomer. This is the dromaeognathous arrangement. In Carinatae 

 (excepting Tinamus} the palatines and pterygoids articulate with the 



* P. Z. S. 1867. 



