94 LAND MAMMALS IN THE WESTERN HEMISPHERE 



typical formula for the higher terrestrial mammals above the 

 grade of the marsupials and which is but rarely exceeded, is 

 ih CiJ pI, 'fn^, X 2 = 44, though most existing mammals have 

 fewer teeth than this. Compared with the typical formula, the 

 Dog has lost but two teeth, the third upper molar on each 

 side, while Man and the Sheep have each lost twelve. 



As every one knows from his own experience, mammals 

 normally have two sets of teeth, the first, temporary, or milk- 

 dentition, in the young animal, and the second, or permanent 

 dentition, in the adult. The milk-dentition, when fully 

 developed, consists of incisors, canines and premolars, which 

 usually agree in number, though often not in form, with the 

 permanent teeth which replace them in the adult. The milk- 

 teeth are frequently more conservative than the permanent 

 ones and retain ancestral characters which have disappeared 

 in the adult series, thus affording welcome information as to 

 lines of descent and steps of evolutionary change. While 

 there can be little doubt that the development of more than 

 one dentition, or set of teeth, is the primitive condition among 

 mammals and was derived by inheritance from their lower 

 vertebrate ancestors, in which there was an indefinite succession 

 of teeth ; yet there are many mammals in which the milk- 

 dentition is greatly reduced or altogether lost. In some, the 

 milk-teeth are shed and replaced before birth, in others only 

 the germs of the milk-teeth are formed and never cut the gum, 

 while in others again all traces of the temporary series have 

 vanished. This complete loss of the milk-teeth, like the pres- 

 ence of a great number of simple and similar teeth in the 

 dolphins and porpoises, or the total absence of teeth, as in the 

 anteaters and whalebone whales, is a secondary and derivative 

 condition, never a primitive one. 



The structure of mammalian teeth varies greatly, from the 

 simplest slender cones to enormous and highly complicated 

 apparatus, and, in order to comprehend the significance of 

 these differences, we must look a little more closely into the 



