110 Animal Life 
the upper one bites behind the lower one. The 
teeth in advance of these (of which, except im 
marsupials, the maximum and typical number of 
pairs is three) are the incisors, while those behind 
them, which do not normally exceed seven pairs, 
are the cheek-teeth or molars. In technical descrip- 
Fig. 3. Enlarged outer view of imperfect lower tions if is usual to divide these cheek-teeth into 
DR He nea a molars and premolars, but into such refinements 
BH of classification it 1s unnecessary to enter on the 
present occasion, and it will suffice if we call them all molars. I have said 
that the number of teeth in mammals is usually definite and limited, and that 
there is never more than a single pair of tusks im each jaw. he reader may 
accordingly ask why in the fox’s skull figured at the head of this article there are 
two pairs of these teeth. To this it may be replied that allowance in nature must 
always be made for individual abnormalities, such as four-legged chickens and six-toed 
cats, and that such abnormalities may be inherited. This fox’s skull is a very rare 
and interesting instance of such an abnormality, the two complete upper tusks being 
probably due to the sphtting of the tooth-germ. That such is really the case seems 
to be demonstrated by other abnormalities in fox-skulls (of which there is an example 
in the Natural History Museum), these showing a splitting of the upper extremity of 
one or more of the tusks, clearly due to partial division of the germ. 
The division of the teeth of mammals into incisors, tusks, and molars appears to 
date from very remote antiquity, and is probably inherited from extinct reptilian 
ancestors (the so-called anomodonts of South Africa and elsewhere), in which a similar 
division of the teeth into three groups is noticeable, although no such differentiation 
occurs in modern reptiles. In the earliest mammals, as exemplified by the imperfect 
lower jaw shown in Fig. 3, it is probable that all the molars were of the same 
general type, and usually seven or eight in number, their crowns terminating in sharp 
cusps. Frequently the tusk, or canine, was double-rooted. With the exception of the 
latter feature, such a type of dentition is preserved at the present day by the 
opossums of America (although not by the animals thus miscalled in Australia), and 
by certain other carnivorous or insectivorous marsupials. 
In the great majority of existing mammals the dentition has become more or less 
modified from this primitive type—least so, perhaps, in the carnivorous, and most so in 
the herbivorous. In some cases this modification takes the form of a reduction in the 
number of the teeth, in others by their alteration in 
form and structure, both types of modification not 
unfrequently occurring together. For the most part 
these variations from the typical number and form 
are connected with the food of the various groups 
in which they occur; in other words, they are 
adaptations to function. Im this connection it 
is specially worthy of notice that among the 
herbivorous groups reduction generally takes place 
in the number of the front teeth (incisors and 
tusks), while the cheek-teeth become proportionately 
enlarged and specialised. Of this we have a 
noticeable instance among the ruminating mammals. 
In the carnivorous forms, on the other hand, where 
they are of prime importance in seizing and holding 
the prey, the front teeth, for the most part, undergo Fig. 4, Side view of the Dentition of Lion. 
