What Is a Mammal? 5 
Instead, we look at the American Opossums, the Australian 
Dasyures and Bandicoots, and (not so generalized) at some of 
the tree-shrews, the Flying Lemur, some carnivores and Bats. 
The theoretically basic plan of the teeth of living mammals 
includes four classes of upper and lower teeth. Across the front 
of the mouth is a row of fairly simple single-rooted incisor teeth, 
the ones with which you bite a mouthful out of a slice of bread. 
At the front outer corners of each jaw stands a single, large, 
pointed tooth, the canine tooth. These are exceptionally large in 
most mammals and are used in seizing live prey and in fighting. 
Just behind the canine and extending back to about the angle of 
the lips are some three small, pointed, compressed, blade-like 
teeth, the premolars (bicuspids to dentists), which may act as 
supplementary canines or as supplementary grinding teeth. Be- 
hind them again stand four or less broadened, multicuspid cheek 
teeth, which do most of the chewing of the food — the molars. 
The incisors, canines, and premolars are represented by sets of 
temporary teeth during babyhood and the period of active 
growth; they are replaced by a second, permanent set. The 
molars come into place later, usually from front to back, and 
are not replaced by others. 
In the different orders of mammals this basic pattern has 
become considerably or profoundly altered, probably in response 
to changes in food-getting habits, in the kind of food eaten, 
and in the animals' general mode of life. So long as mammals 
remain meat-eaters or insect-eaters, their teeth tend to keep the 
patterns described above, although some teeth may exchange 
functions with others and the total number of teeth may de- 
crease. But when vegetable matter is added in quantity to the 
diet or replaces meat altogether, very great changes in the teeth 
take place. 
A change of shape in the incisive and canine teeth that has 
taken place in certain still highly insectivorous mammals seems 
to indicate an alteration in their killing procedure to a kind of 
