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,! and Mr. O. 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 pe 
2 ig eg, ee by the loss of the fifth lower incisor, 
OTs # iy, 3, a* "1, 3, 8.4, BP 
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 10,34 1, 2,8, 4. 
L25,40 “Spa e se 
Opossum (Didelphys), usually written i 2, ¢4,p3,m 4. Again, 
in the same formula the loss of the fourth and fifth incisors in 
; : oth 2 8 G0: 
both jaws, and also of the fourth molars, gives us 2 12, 3,0,0 °7’ 
1, 2, 3, 4 1, 2, . A : 
L234 oh, or the formula of a typical Eutherian, like the 
! According to Mr. G. E. Dobson there are four upper incisors in some of 
the Soricide. 
or the formula of the 
