xm PHYLUM CHORDATA 481 



with the exception of the auditory ossicles, the lower jaw, and 

 the hyoid, are all immovably united together by means of sutures. 



The palatine bones develop palatine plates separating off a 

 posterior nasal passage from the cavity of the mouth, a condition 

 found among the Sauropsida only in the Crocodilia, and, to a less 

 extent, in the Chelonia and some Lizards. 



The zygomatic arch, not found in the skull of the Sauropsida, 

 being peculiar to Mammals, is a strong arch of bone formed partly 

 of the squamosal, partly of the jugal and partly of the maxilla, 

 The orbit an the skull of some Mammals is completely enclosed by 

 bone, constituting a well-defined cavity ; in others it is not com- 

 pletely surrounded by bone behind, and so communicates freely 

 with the temporal fossa, which lies behind it. 



The periotic bones (pro-, ofnstk- and epi-otic) become completely 

 fused together in the skull of Mammals. Part of the periotic 

 mass sometimes projects on the exterior at the hinder part of the 

 lateral region of the skull, and is the mastoid portion ; the rest 

 is commonly called the petrous portion of the periotic, and encloses 

 the parts of the internal ear the mastoid portion containing only 

 air-cells. The tympanic bone sometimes only forms a long tube, 

 sometimes a mere ring of bone. In other cases it not only forms 

 a tube for the external auditory meatus, but also forms the bulla 

 tympani, a dilated bony process containing a cavity. 



The occipital region presents two condyles for the articulation of 

 the atlas. 



The mandible consists in the adult of one bone on each side- 

 the two rami, as they are called, being in most Mammals closely 

 united at the symphysis. The mandible articulates with an 

 articular surface, formed for it by the squamosal bone, below the 

 posterior root of the zygomatic arch. 



The hyoid consists of a body and two pairs of cornua anterior 

 and posterior ; of these the anterior pair are longer, and consist of 

 several bones, the most important and most constant of which 

 is the stylohyal, connected usually with the periotic region of 

 the skull. The posterior cornua or thyro-hyals are usually much 

 smaller. 



The ratio borne by the capacity of the cranial cavity to the 

 extent of the facial region varies greatly in the different orders. 

 The greater development of the cerebral hemispheres in the 

 higher groups necessitates a greater development of the corre- 

 sponding cerebral fossa of the cranium. This is brought about by 

 the bulging upwards, forwards, and backwards of the cranial roof, 

 resulting in a great modification in the primitive relations of cer- 

 tain of the great planes and axes of the skull (Fig. 1057). Taking 

 as a fixed base line the basi-cranial axis, an imaginary median line 

 running through the basi-occipital, basi-sphenoid, and presphenoid 

 bones, we find that the great expansion of the cerebral fossa in the 



VOL. II I I 



