Chap, iv.] TEETH OF MAMMALS. 149 



The definite diphyodont arrangement is Lest seen in 

 the higher Mammals. 



The possession of this double series is not, how- 

 ever, the only remarkable character of the teeth of 

 Mammals ; while there are only inconsiderable, if any, 

 differences in the form of the teeth of any given fish 

 or reptile, and such differences are characteristic only 

 of small groups, we find that for a large number of 

 Mammals, though by no means in all, the teeth in 

 different regions of the mouth have distinctly and 

 definitely different forms and function; (1) in the 

 anterior portion we find sharp cutting teeth ; (2) at 

 the sides we sometimes see strong seizing or holding 

 or offensive organs, and, farther back (3) we see that 

 the upper surface of the tooth becomes widened out 

 and tubercnlated so as to form a more or less suitable 

 grinding surface. Looked at in a general way, these 

 three kinds or forms of teeth may be grouped as (1) 

 incisors, (2) canines, or (3) molars. The molars 

 are spoken of in diphyodonts as premolars or 

 molars, according as they are or are not preceded by 

 milk or deciduous molars. Mammals with variously 

 formed teeth are conveniently known as hetero- 

 donts ; while a nomodont dentition is ascribed to 

 such forms as the edentates, or the toothed whales, in 

 which all the teeth have exactly the same character. 



When a homodont dentition obtains, the number 

 of teeth in the jaws may be very great, some dolphins 

 having as many as two hundred (Pontoporia) ; in the 

 other forms the number of teeth is strictly limited, no 

 known living mammal having more than forty-eight 

 teeth (Megalotis). 



In comparing the teeth of one heterodont with 

 those of another, it is very convenient to make use of 

 the set of symbols which make up the "dental for- 

 mula ; " here the letters i, c, pm, and m, represent 

 the different categories of teeth, while the fraction 



