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n CANINES AND PREMOLARS 49 



" liorn " of the Narwhal is another modification of an incisor, as 

 are the tusks of Elephants. Among the Lemurs the incisors are 

 denticulate, and serve to clean the fur in a comb-like fashion. 

 This is markedly the case in Guleo-pilhecus. The incisors arc 

 sometimes totally absent, as in the Sloths, sometimes partial!} 

 absent, as in many Artiodactyles, where the lower incisors bite 

 against a callous pad in the upper jaw, in which no trace of 

 incisors has been found. 



Canine teeth are present in the majority of mammals, but 

 are absent without a single exception from the jaws of the 

 Eodentia. The canine tooth of the upper jaw is that tooth 

 which comes immediately after the suture dividing the pre- 

 maxillary from the maxillary bone. The canines are as a rule 

 simple conical teeth, with but a single root; indeed they 

 resemble what we may presume to have been the first kind of 

 tooth developed in mammals. In this they resemble also as a 

 ;eneral rule the foregoing incisors. But instances are known 

 where the canines are implanted by two roots. This is to be 

 seen in Triconodon, in the pig Hyotherium, in the Mole and 

 some other Insectivores, and in Galeopitliecus, where the incisors 

 also may be thus implanted in the jaw. Furthermore, the 

 simple condition of the crown of the tooth may be departed from. 

 This is the case with a Fruit Bat belonging to the genus Ptera- 

 lopex. In the more primitive Mammalia it is common to find 

 no great difference between the canines and incisors ; such is the 

 case with the early Ungulate types of Eocene times, such as 

 Xiphodon. In modern mammals, however, especially among the 

 Carnivora, the canines tend to become larger and stronger than 

 the incisors, and in some of the Cats and in the Walrus these teeth 

 are represented by enormous offensive tusks. It is not rare for 

 the canines of male animals to be larger than those of their 

 mates. There are also cases such as the Musk-deer and the 

 Kanchil where the male alone possesses these teeth, but only in 

 the upper jaw. The teeth which follow the canines are known 

 as the grinders or cheek teeth, or more technically as premolars 

 and molars. These two latter terms separate teeth which arise 

 at different periods, and their use will be explained later. In the 

 meantime it may be pointed out that the cheek teeth are the teeth 

 which show the greatest amount of variation in their structure ; 

 this is shown by the number and variety of the cusps in which 

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