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Abrocomidae (rat chinchillas; order Rodentia, suborder Hystricomorpha) A family of medium-sized, rat-like rodents that have stiff hairs projecting over the nails of the three central digits of the hind feet. The skull is massive and narrow in the facial region, the brain case is rounded, with low occipital crests and short paroccipital processes, and the cheek teeth are hypsodont. Rat chinchillas live in colonies in burrows or crevices, and can climb trees. They are found only in the Andes from southern Peru to northern Argentina. There are two species in a single genus, Abrocoma.