HIGH SCHOOL ZOOLOGY. 155 



has two sets of teetli, tho milk set and the ponnanent sot : 

 it is diphyodont. Some mammals ai-e monophyodont, they 

 liave only one set of teeth, which ai-e destitute of the fangs 

 or roots of ordinary teeth, and are simply added to in length liy 

 the pulp, as their free surface becomes worn down. Other 

 forms, which are diphyodont with regard to the greater number 

 of their teeth, still retain some with this unlimited power of 

 growth. Although in some reptiles certain teeth are dis- 

 tinguished from tlieir neighbours either by their form or by 

 standing isolated in the series, yet no such specialisation of the 

 teeth in the different parts of the gape occurs, such as we find in 

 the mammals. Here the pi-emaxillary bones lodge teeth of a 

 distinct form, the incisors, which in the cat are six in number, 

 and are separated by a gap or diastema from the large and sharp 

 canines, the foremost teeth of the maxillaries. Corresponding 

 but alternating with these are similar teeth in the lower jaw, 

 but the gaps are in this case behind not in front of the canines. 

 Behind the canines are the premolars, i.e. the teeth which re- 

 place the back teeth of the milk dentition, and behind these the 

 true molars, wliich only appear in the permanent set. Both 

 kinds of grinders are reduced in number in the cat in accord- 

 ance with the principle of specialisation referred to in § 5. 



Thei'e are three premolars and one molar in the upper jaw 

 but only two of these are functional, the first premolar and 

 the molar being evidently rudimentary. On the other hand 

 there are two premolars and one molar in the lower, all three 

 of which are functional teeth. The same alternation which is 

 to be seen further forward in the gape is also to be seen here so 

 that the cat's dental formula of one side might be expressed 

 thus : — 



i i i C pm pm PM m 

 i i i C * })m pm M 



The teeth marked in full-faced type are the characteristic 



