1 66 MORPHOLOGY OF THE ORGAXS OF VERTEBRATES. 



passes through the otic bones (petrosal), the ninth and tenth 

 through the jugular foramen formed by the junction of basi- and 

 exoccipital and otic bones. Occasional variations from these 



conditions occur ; for instance, the optic 

 nerve may pass through a notch in the 

 orbitosphenoid, or, again, the ophthal- 

 mic branch of the fifth ma}' be enclosed 

 in the alisphenoid. 



WTiile the question of the segments 

 of the head will be taken up in a later 

 section of this volume, it may be well 

 to point out here that the bones of the 

 skull form a series of rings surrounding 

 the brain ; but it is to be noticed that 

 these rings are formed in part of mem- 

 brane bones, in part of cartilage bones. 

 The posterior of these rings is formed 

 of basi-, ex-, and supraoccipitals ; next 

 in front comes a ring formed of the 

 basisphenoid, alisphenoids, and parie- 

 tals-; third, one of presphenoid, orbito 

 sphenoids, and frontals ; and lastU', one 

 of ethmoid and nasals. 



In the fishes, stegocephalans, and 

 gymnophiona the membrane bones 

 form a continuous layer on the outside 

 of the skull ; but in the higher verte- 

 brates gaps may occur here and there 

 behind the orbit, the fossae thus formed 

 being bounded by arches of bone. 

 There may be two of these fossae, — 

 a more dorsal supratemporal, and a 

 more ventral and lateral infratemporal. 

 These fossae are separated from the 

 orbit by a bridge of bone, usually consisting of postorbital and a 

 process of the jugal. The infratemporal is bounded externally 

 by a zygomatic arch into which the quadrat ojugal, jugal, and 

 maxillary may enter ; while between the two fossae is an arch 



Fig. 176. Skull of the 

 Dinosaur Hadrosaurus, show- 

 ing supra- (57^ and infra- 

 temporal fossae (/7"), after 

 Cope. FR, frontal ; jf, jugal, 

 X, nasal ; O, orbit ; P, post- 

 orbital ; -PA, parietal ; PM, 

 premaxilla; PDF, post- 

 frontal ; PRF, prefrontal ; Q, 

 quadrate ; 5, squamosal ; SO, 

 supraoccipital. 



