or THE CLASS MAMMALIA. 9 



crowns of^. 3 and p. 4 are of a more simple form than those of the 

 milk-teeth, which they are destined to succeed. When the milk- 

 teeth are shed, and the permanent ones are all in place, their kinds 

 are indicated, in the genus Sus, by the following formula : — 



. 3—3 1—1 4—4 3—3 J J 



^•§=5, 0.—, p.—, m.—=4.4.'. 

 which signifies that there are on each side of both upper and lower 

 jaws 3 incisors, 1 canine, 4 premolars, and 3 molars, making in all 

 44 teeth, each tooth being distinguished by its appropriate symbol, 

 e. g., p. 1 to p. 4, m. 1 to m. 3. This number of teeth is never sur- 

 passed in the placental Diphyodont series. 



When the premolars and the molars are below this typical 

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

 premolar series, and from the back part of the molar series. The 

 most constant teeth are the fourth premolar and the first true 

 molar ; and these being known by their order and mode of deve- 

 lopment, the homologies of the remaining molars and premolars 

 are determined by counting the molars from hefore backwards, e.g. 

 ' one,' ' two,' ' three,' and the premolars from heJiind forwards, 

 * 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 homotype, 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, pre- 

 molars, 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 succeed- 

 ing segment. It suffices, therefore, to reckon and name the teeth 

 of one side of either jaw in a species with the typical 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 characters 

 of the class Mammalia, because they have not been clearly or 

 accurately defined in any systematic or elementary work on zoo- 

 logy, 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. 



I next proceed to review briefly the principal primary divisions 

 of the Mammalia hitherto proposed. The best authorities in 

 Natural History have adopted different characters, drawn from 

 different systems of organs, for the primary groups or divisions of 

 the class Mammalia. 



