Suborder Myomorpha contains 1,137 species of mouse-like rodents, nearly a quarter of all mammal species. Included are mice, rats, gerbils, hamsters, lemmings and voles. They are grouped according to the structure of their jaws and molar teeth. Both their medial and lateral masseter muscles are displaced forward, making them adept at gnawing. The medial masseter muscle goes through the eye socket, a feature unique among mammals.