Herbivores have premolars whose crowns tend to wear to flat grinding surfaces and resemble the more posterior molar teeth. Omnivorous mammals display premolars having a shape in between that of carnivores and herbivores. Humans have only two premolars on each side of the upper and lower jaws. The molars are the posterior-most teeth in the jaw and are the last ones to erupt. In carnivores, these teeth are much flatter and broader crowned than the premolars.
Domestic dogs have three lower, but only two upper molars.
Molars are designed for crushing or grinding. The table below summarizes the common premolar and molar condition in carnivorous and herbivorous placental mammals. Most non-mammals have only one, or occasionally, two types of teeth. The bottom dwelling relatives of sharks, the rays, are mollusc-eaters with flat teeth that enable them to crush the external skeletons of their prey. Most bony fishes have cylindrical teeth which allow them to grasp their food long enough to swallow it.