SKULL OF MAMMALS AND REPTILES 



25 



particulaiiy conspicuous in the Whales and in the Edentates. In 

 the former group the occurrence of the first interceutrum serves 

 to mark the separation of the caudal from the lumbar series. 

 The number of caudals varies from three in Man — and those 

 ([uite rudimentary — to nearly fifty in Manls macrura and Miero- 

 (jafe Jomjica iiihff.a. 



The Skull. — The skull in the Mammalia differs from that 

 of the lower Yertebrata in a number of important features, which 

 will be enumerated in the followino- brief sketch of its structure. 



Fig. 14. — Lateral view of skull of a Dog. C.occ, Occipital condyle ; F, frontal ; 

 F.inf, infra- orljital foramen ; ./</, jugal ; Jm, premaxilla ; L, lacliryiiial ; M , 

 maxilla ; Matid, external auditory meatus ; Md. mandible ; iV, nasal ; ]', 

 parietal ; Pal, palatine ; PJt, process of squamosal ; Pt, pterygoid : Sji/i, ali- 

 splienoid ; Sq, squamosal ; Sq.ccc, supraoccipital ; T, tympanic. (From Wieder- 

 sheim's Com 2iarative Anafomi/.) 



In the first place, the skull i^ a more consolidated whole than in 

 reptiles ; the number of elements entering into its formation is 

 less, and they are on the whole more firmly welded together 

 tlian in Vertebrates standing below the Mammalia in the series. 

 Thus in the cranial region the post- and pre-frontals, the post- 

 orbitals and the supra-orbitals have disappeared, though now and 

 again we are reminded of their occurrence in the ancestors of 

 the Mammalia by a separate ossification corresponding to some 

 of the bones. Nowhere is this consolidation seen with greater 

 clearness than in the lower jaw. That bone, or rather each 

 half of it, is in mammals formed of one bone, the dentary (to 

 which occasionally, as it appears, a separate mento-Meckelian 



