328 



VERTEBRATE ZOOLOGY 



which have no such structures. It is probable that the claw is the 

 most primitive type and that the nail, the hoof, and the naked- 

 fingered type represent a phyletic series of specializations. This 

 idea is more fully discussed in cormection with the mammaUan orders. 

 The Mammalian Skull. — The skull of the mammal (Fig. 168) 

 differs from that of other vertebrates in a number of important par- 

 ticulars. It is more compact and contains fewer elements than that 

 of the reptile. The following bones characteristic of the reptilian 

 ancestry have disappeared from the adult cranium: pre- and post- 

 orbitals, pre- and post-frontals, basipterygoids, quadrato-jugals, and 



.0* B. 



Sq.eee, 



act. 



Fig. 168.— Skull of mammal (dog). C. occ, occipital condyle; F, frontal; 

 F. inf, infra-orbital foramen; Jg, jugal; Jm, premaxilla; L, lachrymal; M, maxilla; 

 M. avd, external auditory meatus; Md, mandible; N, nasal; P, parietal; Pal, 

 palatine; Pjt, zygomatic process of squamosal; Pt, "pterygoid"; SpA, alisphenoid; 

 Sq, squamosal; Sq. occ, supra-occipital; T, tympanic. (From Wiedersheim.) 



supra-temporals. In the lower jaw, the angulare, splenial, and ar- 

 ticulare are gone; but the latter is believed to have been drawn in to 

 form the malleus, one of the ear bonelets. The quadrate has also 

 been drawn in to form the incus bonelet. 



Mammalian Dentition.— The teeth of mammals (Fig. 169) are 

 attached only to the dentary, maxillary, and premaxillary bones. 

 They are limited in number, rarely exceeding fifty-four. The in- 

 cisors are generally simple in structure and with a single root; the 

 canines, when present, are also simple and with a single root; the re- 

 maining teeth (cheek-teeth) are divided into premolars and molars, 

 and show a wide range of complexity in structure and in number of 



