68 MAMMALIAN DENTITION 



from some generalized Polyprotodont form of the Upper 

 Jurassic. 



The most primitive Mammals were probably small insec- 

 tivorous or insectivorous-omnivorous forms with teeth resem- 

 bling those of the Trituberculates and of slow-moving terres- 

 trial or arboreal habit. It may be advantageous at this point 

 to enumerate certain features characteristic of the primitive 

 Mammal and at the same time to state the relationship of these 

 primitive features to the marsupial skull. By comparing the 

 following tables it will be clear that, although modern Marsu- 

 pialia display a greater number of primitive features and may 

 be assigned a longer ancestry than placental Mammals, yet in 

 certain respects the latter present more primitive traits. 



Primitive Characters 



Pound Indifferently in Marsupials and Placentals 



Size: Small. 



Teeth: Adjusted for insectivorous omnivorous diet. 



Incisor rows parallel or obliquely inclined to each other. 



Canines large. 



Premolars increase in size backwards. 



Molars increase in size backwards. 



Milk dentition well represented. 

 Mandible: Condyle sessile; at or about the level of the teeth. 



Coronoid broad. 



Sigmoid notch shallow. 



Angle inflected. 

 Skull: Brain case very narrow. 



Zygomata projecting. 



Sutures open. 



Xasals broader behind. 



Lachrymal extends on to face. 



Features Differentiating Infraclasses of Theria* 



Metatheria (Marsupials) Eutheria (Placentals) 



Teeth : Upper incisors often Upper incisors never more than 3 



more than 3. (except shrews). 



Premolars never more Premolars may be 4. 



than 3. 

 Molars usually 4. Molars rarely more than 3, (Ceta- 



cea, Armadillos, Otocyon). 



*It is not to be supposed that each of these characteristic features is to be found in 

 every example of marsupial or of placental Mammals. 



