250 COMPARATIVE ANATOMY 



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 represented. Thus the adult dental 

 formula of those animals, the teeth of which are represented in Figs. 196 to 

 200, would be 



Fig. 196. Dog, l'.\\\\l = 42 



3*1*3*3 



,, 197. Hedgehog, 2 . i 2~3 = ^ 6 



,, .198. Porcupine, ^ . Q . ^ . 3 = 20 



3 ' 3 

 199. Sheep, g . 1 . 3 ~ 3 =32 



2*1 ' 2 * 3 

 ,, 200. Catarrhine Monkey, OVVTJTTQ ~ ^ 



The most complete dentition is seen amongst Marsupials, the dental formula 

 of Myrmecobius being 3.1.3.5 g = 50 52. The more typical arrange - 



3-1-4-3 



ment is 3 . x . 4 . 3 - 44. 



Sexual differences in dentition exist in a number of Mammals. Thus in 

 the male Wild Boar, Narwhal (Monodon), Dugong (Halicore), and Musk-deer 

 a modification 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 Ornithorhynchus the teetli become replaced functionally after 

 a time by the development of horny masticatory plates, 1 and in 

 Echidna they are wanting altogether. Adult Whalebone Whales and 

 certain Edentates (Myrmecophaga, Manis) are toothless, but rudi- 

 ments of teeth exist in the embryo. In other Edentates the teeth 

 are wanting in enamel. Canines are absent in certain Mammals 

 (e.g., Rodents) and the incisors may also be wanting. In the 

 typical Ruminants incisors and canines are present in the lower 

 jaw only. 



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. 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. 



With their gradually increasing physiological importance a 



1 Horny crushing plates are also present in the Sirenia, the existing forms of 

 which possess numerous teeth, while the extinct Rhytina was toothless. 



