324 COMPARATIVE ANATOMY 



Sexual differences in dentition exist in a number of Mammals. 

 Thus in Apes the canines and first premolars are more strongly 

 developed in the male than in the female. In the male Wild Boar, 

 Narwhal (Monodon), Dugong (Halicore), and Musk-deer a modifi- 

 cation of certain of the teeth (the canines or the incisors) to form 

 tusks occurs, and these serve as fighting weapons. In the Elephant 

 and Walrus tusks are present in both sexes : in the former they 

 correspond to incisors, and in the latter to canines. In Elephas 

 antiquus they were relatively enormous. 



In describing the teeth of a Mammal it is convenient to make 

 use of a dental formula in which their number and arrangement 

 can be seen at a glance, the teeth of one side only being repre- 

 sented. Thus the adult dental formula of those animals, the teeth 

 of which are represented in Figs. 234 to 237, would be : 



The most complete dentition is seen amongst Marsupials, the 



dental formula of Myrmecobius being ' t - = 52 54. 



o * 1 ' 3 * 5 or u 



The more typical arrangement is ^ - = 44. 



O * I * 4: ' O 



Glands of the Mouth. 



The glands of the mouth, like those of the orbit and integu- 

 ment, appear first in terrestrial Vertebrates, that is from Amphi- 

 bians onwards. 1 They have the function of keeping moist the 

 mucous membrane which comes into contact with the outer air. 

 From being at first almost entirely unspecialised, and giving rise 

 simply to a slimy fluid, they become differentiated later into 

 structures the secretions of which take on a very important function 

 in relation to digestion ; they may also, as in the case of poisonous 

 Snakes and Lizards, constitute dangerous weapons of offence. 



J A paired glandular organ (so-called "salivary gland") is present in 

 Petromyzon between the mouth and first pair of gill apertures, towards the 

 ventral side. It is surrounded by muscles, apparently produces a fatty secretion, 

 and opens into the mouth. 



