All mammals except the monotremes, the edentates, the pangolins, and the cetaceans have up to four distinct types of teeth, with a maximum number for each. These are the incisor (cutting), the canine, the premolar, and the molar (grinding). Mammals that have distinct types of teeth are heterodont; others are homodont.
The number of teeth of each type is written as a dental formula for one side of the mouth, with the upper and lower teeth shown on separate rows. The number of teeth in a mouth is twice that listed as there are two sides. In each set, incisors are indicated first, canines second, premolars third, and finally molars. For example, the formula 18.104.22.168 for upper teeth indicates 2 incisors, 1 canine, 2 premolars, and 3 molars on one side of the upper mouth.
The dentition can be expressed as a dental formula. Teeth are numbered starting at 1 in each group, except the premolars which end at 4. This means that the carnassials are always the fourth upper premolar and the first lower molar. Thus the human teeth are I1, I2, C1, P3, P4, M1, M2, and M3. The human dental formula is:
Of cats it is:
The last upper premolar and first lower molar of the cat, since it is a carnivore, are called carnassials and are used to slice meat and skin. The armadillo, being homodont, has a dental formula that is simply 7/7.
The maximum dental formula for placental mammals is:
In many mammals the children have a set of teeth that fall out and are replaced by adult teeth. These are called deciduous teeth, baby teeth or milk teeth. Animals that have two sets of teeth, one followed by the other, are said to be diphyodont. Normally the formula for milk teeth is the same as for adult teeth except that the premolars are missing.
The milk tooth formula for humans is:
Dentition is particularly useful in tracking ancient populations' movements, because, although all humans have the same basic 32 teeth, there are subtle differences in the shapes of incisors, the number of grooves on molars, and extra cusps on particular teeth. These differences can not only be associated with different populations across space, but also change over time so that the study of the characteristics of teeth could say which population one is dealing with, and at what point in that population's history they are.