44 MAMMALIAN DENTITION 



possibly to become auditory ossicles the malleus and incus 

 respectively,* and the dentary itself forms with the glenoid 

 fossa on the squamous temporal a new and secondary articula- 

 tion which tends to lie above the plane of the cheek teeth and 

 not at or below their level as in Reptiles. In the Alligator 

 some teeth project more than others although all are alike in 

 shape. In the earliest Mammals it is always the first maxillary 

 tooth which takes on a caniniform character, the incisors are 

 relatively simple and the postcanine teeth become increasingly 

 complex backwards. AVhereas all reptilian teeth possess only 

 a single fang, the roots of the cheek teeth and occasionally of 

 the incisors and canines in the Mammal are subdivided. 



In the skull as in the jaws we shall refer only to certain 

 features of special present significance. In the Reptile the 

 anterior nares are completely surrounded by bone ; in the 

 Mammal they are not. The reptilian condition is largely due to 

 an upgrowth of bone from the premaxilla forming an osseous 

 narial septum to which is attached the egg tooth utilized by the 

 hatching Reptile for the purpose of chipping open the egg.f 

 In Mammals the orbit is separated from the temporal fossa by 

 the postorbital process of the frontal and not by a special bone, 

 the postorbital, as in Reptiles. The temporal fossa and coro- 

 noid process of the mandible providing attachments for the 

 temporal muscle, and the zygomatic arch and ramus of the 

 mandible for the attachment of the masseter are well devel- 

 oped in typical Mammals. The hard palate in the Reptile is 

 formed by the premaxillae alone so that the posterior nares 

 open into the mouth. In the Mammal on the other hand the 

 horizontal processes of the maxillary and palate bones forming 

 a septum between mouth and nose and thus forcing the latter 

 to open into the pharynx, constitute the so-called secondary 

 hard palate behind the incisive or anterior palatine foramen. 



*It must not be supposed that all writers are in agreement concerning the fate of 

 these elements. What we do know is that the elements disappear as such. The malleus, 

 incus, tympanic ring, interarticular cartilage of the temporo-mandibular joint and even 

 the zygomatic process of the squamous temporal have been claimed as representing the 

 quadrate. 



f This bone is represented in the young Monotremes and, according to Broom, in the 

 young of some Marsupials (Macropus, Trichosurus). 



