DENTAL SYSTEM 27 



form and function they represent in a condensed form the whole 

 premolar and molar series of the adult. When there is a marked 

 difference between the premolars and molars of the permanent 

 dentition, the first milk-molar resembles a premolar, while the last 

 has the characters of the posterior true molar. 



The dentition of all the members of the orders Primates, 

 Carnivora, Insectivora, Chiroptera, and Ungulata can clearly be 

 derived from the above -described generalised type. The same 

 may be said of the Rodents, and even the Proboscidea, though 

 at least in the existing members of the order with greater modi- 

 fication. It is also apparent in certain extinct Cetacea, as 

 Zeuglodon and Squalodon, but it is difficult to find any traces of 

 it in existing Cetacea, Sirenia, or any of the so-called Edentata. 

 All the Marsupials, different as they are in their general structure 

 and mode of life, and variously modified as is their dentition, 

 present in this system of organs some deep-lying common characters 

 which show their unity of origin. The generalised type to which 

 their dentition can be reduced presents considerable resemblance 

 to that of the placental mammals, yet differing in details. It is 

 markedly heterodont, and susceptible of division into incisors, 

 canines, premolars, and molars upon the same principles. The 

 whole number is, however, not limited to forty-four. The incisors 

 may be as numerous as five on each side above, and they are 

 almost always different in number in the upper and the lower jaw. 

 The premolars and molars are commonly seven, as in the placental 

 mammals, but their arrangement is reversed, as there are four 

 true molars and three premolars. 



The larger number of incisive and molar teeth among the 

 Marsupials suggests that their additional teeth have disappeared 

 in the Eutheria, 1 and Mr. 0. Thomas has endeavoured to construct 

 a generalised dental formula from which both the Marsupial and 

 Eutherian modifications may have been derived by the suppression 



of particular teeth. Thus the hypothetical formula i^ 



m ~~> ky the l ss f the fifth lower incisor, 



i' i 2 3 4' i 2 3 

 and of the second premolars (which we know to be those which 

 disappear in the Marsupials) and the fifth molars, will give 



1, 2, 3, 4, 5 1 1, 0, 3, 4 1, 2, 3, 4 ., , , , ., 



* i, 2, 3, 4, o> c I' P 170737!' m ITvsTV or the formula of the 

 Opossum (Didelphys), usually written i J, c^,p ^, m . Again, 

 in the same formula the loss of the fourth and fifth incisors in 



12300 1 

 both jaws, and also of the fourth molars, gives us i 1 ' 2 ' 3 ' Q ' Q> c -, 



P !' ' Q' !> m . r^ 1 !. or the formula of a typical Eutherian, like the 



I . . -'. 4 lj 4) O 



1 According to Mr. G. E. Dobson there are four upper incisors in some of 

 the Soriddce. 



