ORAL EQUIPMENTS. 55 



" Homodont " order, we find some wonderful specialisations 

 of the teeth ; and confining myself for the present to 

 those of the incisor series, I will take as an example the 

 u Narwal." 



By the diagram you will perceive there is an enormously 

 developed tusk on one side of the upper jaw, and on the other 

 side one of the same form, but very rudimentary. These are 

 the only teeth the creature possesses. 



In the female they are small, about eight inches, and 

 never come through the bone at all ; but remain enclosed 

 within its substance in a merely rudimentary condition ; but 

 in the male one of them (and in some very rare instances 

 both) continues to grow till it has attained the enormous 

 length of ten or twelve feet, and a diameter of three or four 

 inches at its base. 



This presents a well-marked instance of a sexual weapon 

 developed in the incisor series, rather a rare occurrence, as 

 generally it is the canine _ which is the object of sexual 

 selection. This enormous tusk (generally, or rather nearly 

 alwa}^s, the left one) is quite straight, but is marked by spiral 

 grooves winding from right to left ; and it is a curious fact 

 that in those specimens whose tusks have both attained 

 functional proportions, the grooves wind in the same direction. 

 The spiral form is that which affords the maximum strength 

 and elasticity. 



I shall have more to say with reference to the Cetacea 

 later on when describing the " Baleen or Whalebone Whale." 

 At present we will continne to consider the differentiation of 

 the incisor teeth. 



The Elephant and his primaeval ancestors, the Mammoth, 

 Dinotherium, Megatherium, and Mastodon offer marked 

 instances of this. 



In the Elephants it is the upper incisors, and they are 

 lateral incisors which are differentiated, and it is in the male 

 that they attain to their maximum development. Another 

 instance of sexual selection, though in the female also 

 they are no mean weapons. They are used for rooting 

 up trees and shrubs, &c, to get at the herbage on top 

 branches which they cannot reach with their trunks. • In the 

 African Elephant the tusks attain a greater length than in 

 its Indian cousin. 



The molar teeth of these creatures offer some very 

 interesting points for consideration, which will be referred 

 to in the order I have adopted for description. 



The Hippopotamus has well specialised front teeth, which 



