ORGANS OF DIGESTION 1 33 



the end of the second month. At the end of the fourth month 

 the milk incisors are being displaced by the permanent teeth. 

 The formula for the temporary set is di J^, dc Jf , dm ^^. 



Teeth are present in nearly all adult mammals. The whale- 

 bone 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 con- 

 dition. The milk teeth are present, but none become sufh- 

 ciently 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, pre- 

 molars, 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 mam- 

 mals with a heterodont dentition the number of teeth in the 

 different genera varies considerably, as seen by the following 

 formula: 



Pig iH,cH,pH,mH =44 



Dog iH,c}i,p H,^H =42 



Cat i ^, c H, P M, m K =3° 



Porcupine \ }{, c %, p }{> ^ H = 20 



Sheep i %,cH,p%,m% =32 



Horse i H, c }i, p H, i^ H =44 



Opossum i H, c Vi, p H, m ji = so 



Man i M, c H, P %, m /i =32 



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 through out the entire life of the animal. 



