ORGANS OF DIGESTION. m 



that it is larger and presents an anterior basal cusp in addi- 

 tion to the others. The lower molar, sometimes called the 

 sectorial, because it shears against the upper sectorial or 

 carnassial premolar, presents two nearly equal cusps, the 

 protoconid and the paraconid. The root consists of two 

 fangs, the anterior of which is the larger. 



The deciduous or milk teeth begin to appear above the 

 gums when the kitten is two weeks old. The incisors and 

 the canine appear first, then the second and third molars. 

 The first upper molars do not appear till the kitten is about 

 six weeks old. According to Jayne, the deciduous dentition 

 is complete at 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 %, dc %, dm %. 



Teeth are present in nearly all adult mammals. The 

 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 perma- 

 nent 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, ex- 

 cept the last premolar, and in some cases the canine and the 

 incisors. The sloths have only one set of teeth and are 

 therefore Monophydont. Mammals having both a milk and 

 a permanent set are termed Diphydont. 



In the majority of mammals the teeth are divided accord- 

 ing 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 heterodont dentition the 



