20 



When the premolars and the molars are below this typical 

 number, the absent teeth are missing from the back part of 

 the molar series, and usually from the fore part of the pre- 

 molar series. The most constant teeth are the fourth premolar 

 and the first true molar. These being known by their order 

 and mode of development, the homologies of the remaining 

 molars and premolars are determined by counting the molars 

 from before backwards, e. g. ' one,' ' two,' ' three,' and the pre- 

 molars from behind forwards, l four,' ' three,' ' two,' ' one.' 

 The incisors are counted from the median line, commonly the 

 foremost part, of both upper and lower jaws, outwards and 

 backwards. The first incisor of the right side is the homo- 

 type, transversely, of the contiguous incisor of the left side in 

 the same jaw, and vertically, of its opposing tooth in the 

 opposite jaw ; and so with regard to the canines, premolars, 

 and molars ; just as the right arm is the homotype of the left 

 arm in its own segment, and also of the right leg of a suc- 

 ceeding segment. It suffices, therefore, to reckon and name 

 the teeth of one side of either jaw in a species with the typi- 

 cal number and kinds of teeth, e. g. the first, second, and 

 third incisors, the first, second, third, and fourth premolars, 

 the first, second, and third molars ; and of one side of both 

 jaws in any case. 



I have been induced to dwell thus long on the dental cha- 

 racters of the class Mammalia, because they have not been 

 rightly defined in any systematic or elementary work on 

 zoplogy, although an accurate formula and notation of the 

 teeth are of more use and value in characterizing genera in 

 this than in any other class of animals. 



Mammals may be surpassed in the rapidity with which 

 the blood circulates, in the extent and completeness of the 

 respiratory processes,' in bodily temperature, in the concomi- 

 tant vigour of the muscular actions ; all which superiorities, 

 in Birds, for example, result in those marvellous powers of 

 flight with which the feathered class is privileged. But in 

 their psychical phenomena the Mammalia, as a class, excel all 



