Adult humans have twelve molars, in four groups of three at the back of the mouth. The third (rearmost) molar in each group is called a wisdom tooth. It is the last tooth to appear, breaking through the surface of the gum at about the age of twenty, although this varies by ethnicity.
The types of molars in the human mouth are:
maxillary first molars, maxillary second molars, and maxillary third molars
Molars differ considerably from one species to another, so there are many terms describing them:
Tribosphenic: This kind is found in insectivores and young platypuses (adults have no teeth). Upper molars look like three-pointed mountain ranges; lowers look like two peaks and a third off to the side.
Quadrate: This kind is found in humans and various other species. Four cusps are arranged in a rectangle; there may be a fifth.
Bunodont: The cusps, instead of being sharp peaks, are rounded hills. The entire tooth is covered in enamel, and is most common among omnivores such as the pig, the bear and humans.
Hypsodont: There is a lot of enamel and dentine above the gumline and the top of the pulp. This kind of molar is found in mammals that wear their teeth a lot, such as the horse.
Zalambdodont: The tooth has two ridges that meet at an angle, forming the letter lambda.
Dilambdodont: Like zalambdodont, but there are two lambdas on one tooth.
Lophodont: The tooth has a few ridges perpendicular to the jaw.
Selenodont: The tooth has a crescent-shaped ridge or ridges.
Loxodont: The tooth has several parallel oblique ridges on its surface. The elephant Loxodonta is named for this feature.
The molar design that is considered one of the most important characteristics of mammals is a three-cusped shape called a tribosphenic molar. This design of molar has two important features: the trigonid, or shearing end, and the talonid, or crushing heel. With the exception of Jurassic mammal Shuotherium, the talonid is posterior the trigonid.
The tribosphenic design appears in all species of mammals. In monotremes, it seems to have developed independently, rather than from common ancestry with marsupials and placentals.