ORGANS OF DIGESTION. 123 



whalebone whales, the duck-bill, and some of the ant- 

 eaters have no teeth in the adult state, but teeth are 

 present during their embryonic life. The Echidna shows 

 no evidence of teeth at any time. Most mammals have 

 two distinct sets of teeth, known as the milk or deciduous 

 set, and the permanent set. In the Marsupialia, the 

 milk dentition is in a degenerate condition. The milk 

 teeth are present, but none become sufficiently developed 

 to appear above the gums, except the last premolar, and 

 in some cases the canine and the incisors. The sloths 

 have only one set of teeth and are therefore Monophy- 

 dont. Mammals having both a milk and a permanent 

 set are termed Diphydont. 



In the majority of mammals the teeth are divided 

 according to form and function into four groups: incisors, 

 canines, premolars, and molars. Such a dentition is 

 known as Heterodont, in distinction to the Homodont 

 dentition, in which all the teeth have the same form, as 

 is the case in the dolphins. In mammals with a hetero- 

 dont dentition the number of teeth in the different genera 

 varies considerably, as seen by the following formula : 



m I =44 



m \ -- 42 



m ^ = 30 



P I, m | =20 

 f , m = 32 

 ?, m =44 

 |, m | =50 



f P I, m f 



The elephant presents a very specialized dentition. It 

 has no canines nor any lower incisors. The single pair 

 of upper incisors is developed into long tusks, much 

 prized for ivory. They continue to grow throughout 

 the entire life of the animal. Six molars are present 

 on each side, only one or two of which are functional at 



