66 MAMMALIAN DENTITION 



themselves to the maternal teats, remaining thus until capable 

 of looking after themselves until which time they are nourished 

 by injections of milk induced by periodic contractions of the 

 breast musculature. The pouch is really a mere fold of abdom- 

 inal skin and is temporarily or permanently present according 

 to the species. The Marsupial represents a very ancient type 

 of Mammal probably older than the Placental. 



In addition to the distinguishing characters relative to birth 

 and the genital organs Marsupials present other features some 

 of Avhich we may note as differentiating them from placental 

 Mammals. Of these the inflection or inbending of the mandibu- 

 lar angle is a character present in all but one of the Marsu- 

 pials although occurring to a less extent in the Insectivores 

 and in a few Rodents. The condyle is low and sessile and the 

 sigmoid notch shallow. The brain case is relatively narrow 

 and in consequence the zygomatic arches project very ob- 

 viously. The sutures tend to remain open long after the animal 

 reaches adult life. The nasal bones increase in breadth back- 

 ward. The malar or zygomatic forms part of the glenoid fossa. 

 The tympanic remains throughout life a simple ring unfused 

 with surrounding bones. The great wing of the sphenoid forms 

 part of the tympanic bulla. Fenestra? or openings occur in the 

 hinder part of the palate. Not all the foregoing features are 

 truly primitive but taken together they clearly distinguish 

 the skull of a Marsupial from that of a placental Mammal. 



In the Marsupial there may be as many as five incisor teeth 

 m each side of the upper jaw whereas in a placental Mammal 

 there are rarely more than three. The incisor teeth in the 

 Marsupial usually differ in number in upper and lower jaws. 

 The premolars are never more than three in number, the 

 molars usually four and sometimes more/* In Placentals four 

 premolars and three molars represent the maximum. f A fur- 



*In order to bring the Marsupials into harmony with placental Mammals some 

 writers describe the first molar as a premolar, thus attributing to the Marsupial four 

 premolars and three (or more) molars. The older usage is followed in this volume. 



fExcept in a few Placentals, for example the Large-eared Wolf of South Africa 

 (Otocyon) which has four lower molars on each side. In some Armadillos and in 

 some Cetacea the postcanine teeth are increased in number. 



