The order Rodentia consists of small to medium sized mammals which share the common characteristic of a single pair of upper and lower incisors that are specialized for gnawing. Rodentia is the largest order of living mammals, containing approximately 1500 species. The skulls of rodents vary in length from .25 inch to over 10 inches long. The tooth count does not exceed 22 except in the Mole Rat which has 28 teeth.
All rodents have incisors that grow continuously throughout their lives.
The small order Chinchillidae consists of only six species of viscachas and chinchillas. They are found only in western and southern South America. Many have enlarged auditory bullae which increase their ability to evade predators. They are vegetarian, gregarious, and mainly active through all times of the year. The order Rodentia consists of small to medium sized mammals which share the common characteristic of a single pair of upper and lower incisors that are specialized for gnawing.