IV INTRODUCTION 



in the foetal state, and are either shed before birth, or very shortly- 

 after. The last of the premolars is called the flesh-tooth, or carnas- 

 sier, or scissor-tooth, as the upper one acts on the lower one like a 

 pair of scissors. It is greatly developed in the Cat tribe. 



The true molars never exceed three in number on each side, 

 above and below, and always end in two, three, or even four roots 

 or fangs, sometimes considerablj' divergent from each other. They 

 vary much in form and size, and are present in all Mammals except 

 a few Ant-eaters, Whales, and the curious Duck-billed Flatypm. 

 They have four tubercles in most Monkeys, two sharp-pointed 

 tubercles in Shrews and other Insectivorous animals ; are three- 

 pointed in some, and conical or flat in others. They are more or 

 less numerous according to the herbivorous or carnivorous nature of 

 the animal, there being only one on each side in the Feline tribe. 



In the Elephant there is only one molar on each side, above and 

 below, and this forms a seeming exception to the non-renewal of 

 of these teeth, for as it gets worn away another is developed pos- 

 teriorly, and gradually pushes the other out, so that there are 

 sometimes two on one side at a time, but never more. The typical 

 number of teeth is considered to be 44, and this number is never 

 exceeded except in the fish-like Cetacea, and a few others of rep- 

 tilian afiinities. These teeth are arranged thus : Incisors 5 — x = 



1 1 4 4 3 3 



12; Canines =— -:: = 4; Prsemolars — -r = 16; Molars = — = = 



12. 



The mouth of Mammals is surrounded by fleshy lips, more or less 

 protusile, and the cheeks in some form pouches for the reception 

 of food taken hurriedly. 



The extremities vary more than in Birds, in length, form and 

 structure, as noticed previously. The nails or claws are useful 

 guides to assist in classifying Mammals. In some they are blunt 

 and terminal as in Monkeys ; in others sharp as in Shrews ; much 

 curved, powerful, sharp and retractile in Cats ; long and strong, and 

 well adopted'for digging (fossorial), as in the Scaly Ant-eaters and 

 Bears. In the herbivorous animals they are solid as in the Horse 



