702 



MAMMALIA. 



The skull consists, as in all the higher Vertebrates, of two 

 sets of bones, cartilage bones preformed in the cartilage of 

 the original gristly brain-box and its associated arches, and 

 membrane bones developing in the investing membrane and 

 not preformed in cartilage. (The names of the membrane 

 bones are printed in italics.) 



We have already noticed the chief characteristics of the 

 mammalian skull, such as the usual persistence of sutures, 



the two condyles, the bony 

 palate, the fusion of the 

 periotic bones, the articu- 

 lation of the mandible with 

 the squamosal, the fusion 

 of the parts of each ramus 

 of the mandible into a single 

 bone in the adult, and the 

 three ossicles of the ear. 



The foramen magnum is 

 bounded by the basioccipital 

 beneath, the exoccipitals on the 

 sides, the supraoccipital above. 

 The exoccipitals form most of 

 the occipital condyles, but the 

 ~~Z.SQ basioccipital contributes a small 

 part. In many Mammals the 

 exoccipitals alone form the con- 

 dyles. From each exoccipital a 

 paroccipital process descends, 

 and is applied to the tympanic 

 bulla a dilatation at the base 

 of the tympanic bone which pro- 

 tects the external auditory tube. 



Along the roof of the skull 

 lie the supraoccipital, the inter- 



, parietal* the parietals. thefren- 

 W., Anterior nostril ; /W.<V.,premaxilla; NA., \ , A ^ / 



nasal ; FR., anterior part of frontal ; MX., tals > and tne * 

 posterior part of maxilla; /., anterior part On the very front of the skull 

 of jugal; SO. F., supraorbital process of fron- are the premaxillce, bearing the 

 tal; FRR., posterior part of frontal; //., : nr ,: cnr t^fVi "R^hi'nH ^rh fir* 

 posterior end of jugal protruding below zygo- mciso f teeth. J 1 CaCtt/W- 



matic portion of squamosal (Z.SQ.); PA., maxilla is a maxilla, bearing 

 parietal; AM., external auditory meatus; the premolars and molars; be- 

 SO., supraoccipital; IP., interpanetal; ^., hind thiSj along the Z y gO matic 



JJ 



M 



FIG. 384. Upper surface of 

 rabbit's skull. 



squamosal. 



or temporal arch projecting be- 



neath the orbit, is the jugal or malar , which unites posteriorly with 

 the squamosal. This zygomatic arch bridges over the deep temporal 

 fossa behind the orbit, and serves for the insertion of muscles, and its 

 "squamoso-maxillary " structure occurs outside of Mammalia in the 



