II CANINES AND PREMOLARS 49 
“horn” 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 Guleopithecus. The incisors are 
sometimes totally absent, as in the Sloths, sometimes partially 
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 im the majority of mammals, but 
are absent without a single exception from the jaws of the 
Rodentia. 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 
general rule the foregoing incisors. But instances are known 
where the canines are implanted by two roots. This is to be 
seen in TZriconodon, in the pig Hyotheriwm, mm the Mole and 
some other Insectivores, and in Galeopithecus, 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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