THE EONES OF TUE EACE. 27 



whicli un(]erlics it from tlie basi-occipital to the pre-splienoidal 

 region. In ordinary fishes and Amjihibia, this bone appears 

 to replace the basisphenoid and presphenoid functionally, 

 while in the higher Vertebrata it becomes confounded with 

 the basisphenoid. The Vomer is a similar, splint-like, single 

 or double, membrane bone, which, in like manner, underlies 

 the ethmoid region of the skull. 



In addition to the bones already mentioned, a prefrontal 

 l)one may be developed in the prefrontal region of the nasal 

 capsule, and bound the exit of the olfactory nerve externally. 



A postfrontal bone may appear behind the orbit above the 

 alisphenoid. Sometimes it seems to be a mere dismember- 

 ment of that bone ; but, in most cases, the bone so named is a 

 distinct membrane bone. 



Furthermore, on the outer and upper surface of the audi- 

 tory capsule a membrane bone, the squamosal, is very com- 

 monly developed ; and another pair of splint-bones, the nasals, 

 cover the upper part of the ethmovomerine chambers, in 

 which the olfactory organs are lodged. 



The Osseous Facial Apparatus. — The bones of the face, 

 which constitute the inferior arches of the skull, appear with- 

 in the various processes and visceral arches which have been 

 enumerated. Thus, the premaxillm are two bones developed 

 in the oral part of the naso-frontal process, one on each side 

 of the middle line, between the external nasal apertures, or 

 anterior nares, and the anterior boundary of the mouth. 



Ossification occurs in the palato-pterygoid cartilage at two 

 chief points, one in front and one behind. The anterior gives 

 rise to the palatine bone, the posterior to the j)terygoid. 

 Outside these, several membrane bones may make their ap- 

 pearance in the same process. The chief of these is the max- 

 illa, which commonly unites, in front, with the premaxilla. 

 Behind the maxilla there may be a second, the jugal ; and 

 0(>casionally behind this lies a third, the quadrato-jugal. 



Between the maxilla, the prefrontal and the premaxilla, 

 another membrane bone, called lachrymal, from its ordinary 

 relation to the lachrymal canal, is very generally developed ; 

 and one or more suprororhital and post-orhital ossifications 

 may be connected veith the bony boundaries of the orbit. 



When these and the postfrontal membrane bone are si- 

 multaneously developed, they form two series of bony splints 

 attached to the lateral wall of the skull, one set above and 

 one below the orbit, which converge to the lachrymal. The 



