Rodents have enlarged, chisel-shaped upper and lower front incisors that grow throughout their lives. These have hard enamel on the front surface and soft dentine on the back surface, so that unequal wear keeps the chisel edge sharp. There is a gap between the front teeth and the cheek teeth. When the lower jaw is in a forward position, for gnawing, the upper and lower incisors are in contact but the upper and lower cheek teeth are not; thus, wear on the cheek teeth is avoided. The cheeks are drawn in behind the incisors when the animal is gnawing, so that bits of hard material cannot be swallowed. When the lower jaw is pulled back into the chewing position, only the cheek teeth make contact.
The approximately 4,000 rodent species are divided on the basis of their anatomy into three well-defined groups, or suborders, and more than 30 families. The Sciuromorpha, or squirrellike rodents, include the various species of squirrel, chipmunk, marmot, woodchuck (or ground hog), prairie dog, gopher (or pocket gopher), pocket mouse, kangaroo rat, and beaver. The Myomorpha, or mouselike rodents, include a great variety of mouse and rat species, as well as species of hamster, lemming, vole, muskrat, gerbil, dormouse, and jerboa. This is the largest rodent group. The Hystricomorpha, or porcupinelike rodents, include the porcupine, capybara, nutria (or coypu), agouti, cavy (including the domestic guinea pig), mara, and chinchilla, as well as many species whose common names include the term rat (e.g., the South American bush rat).
Rabbits and hares were once classified as rodents because of their large, chisel-shaped incisors. However, they are quite distinct anatomically and have a long, separate evolutionary history; they are now classified in an order of their own, the Lagomorpha. Using DNA analyses as evidence, some scientists believe that the some other groups of rodents have descended from different ancestors and should thus be placed in orders of their own.
See Sir J. R. Ellerman, The Families and Genera of Living Rodents (2 vol., 1940, repr. 1965); B. S. Vindgradov and A. I. Argiropulo, Key to Rodents (tr. 1968).
Rodent's Revenge is a puzzle video game, created in 1991 by Christopher Lee Fraley. It was distributed as part of the Microsoft Entertainment Pack. The player takes on the role of a mouse, with the objective being to trap cats by pushing blocks around, whilst avoiding obstacles.
There are 50 levels, which get increasingly harder, but the player is allowed to start playing at any level. The difficulty of each level is determined by several factors. Sink holes trap the mouse for a few seconds. Mouse traps will kill the mouse if it accidentally walks into one. Flying Balls of Yarn will kill the mouse on contact. The number of Movable Blocks will determine how difficult it is to trap the cats, while the Unmovable Blocks make it hard to move blocks around and hinder navigation.
During each level a clock ticks away until it reaches a certain point, which is represented by a blue line. When this happens more cats are spawned into the level, making it more difficult, but also increasing the potential reward. After the new cats have spawned, the blue line moves further around the clock and it resumes ticking.