790 



PHYLUM CHORDATA : CLASS MAMMALIA 



not preformed in cartilage. (The names of the membrane 



bones are printed in itaUcs.) 



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 

 basioccipital contributes a small 

 part. In many Mammals the 

 exoccipitals alone form the con- 

 dyles. From each exoccipital 

 a paroccipital process descends, 

 and is apphed to the tympanic 

 bulla — a dilatation at the base 

 of the tympanic bone which 

 protects the external aiiditory 

 tube. 



Along the roof of the skull 

 lie the supraoccipital, the in- 

 terparietal, the parietals, the 

 frontals, and the nasals. 



On the very front of the 

 skull are the premaxilla, bear- 

 ing the incisor teeth. Behind 

 each premaxilla is a maxilla, 

 bearing the premolars and 

 molars ; behind this, along the 

 zygomatic or temporal arch pro- 

 jecting beneath the orbit, is the jusal 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 Mammaha 

 in the Anomodont reptiles only. The fact that in Rodents the m.alar 

 does not form part of the face is of considerable systematic importance. 



Fig. 473. — Upper surface of 

 rabbit's skull. 



N., Anterior nostril ; PMX., premaxilla ; 

 NA., nasal ; FR., anterior part of frontal ; 

 . MX., posterior part of maxilla ; /., 

 anterior part of jugal ; SO.F., supraorbital 

 process of frontal ; FRR., posterior part of 

 frontal ; //., posterior end of jugal pro- 

 truding below zygomatic portion of 

 squamosal {Z.SQ.) ; PA., parietal ; A.M., 

 external auditory meatus ; SO., supra- 

 occipital ; IP., interparietal ; SQ., squa- 

 mosal. 



