On the arid plateaus of the north-west, the guanaco and vicuna are still to be found, though less frequently, together with a smaller species of viscacha (Lagidium cuvieri).
The timid viscacha (Lagostomus trichodactylus), living in colonies, often with the burrowing owl, and digging deep under ground like the American prairie dog, was almost the only quadruped to be seen upon these immense open plains.
In the squirrels and porcupines the tibia and fibula are distinct, but in rats and hares they are united, often high up. The hind foot is more variable than the front one, the digits varying in number from five, as in squirrels and rats, to four, as in hares, or even three, as in the capybara, viscacha and agouti.
With the cheek-teeth formed of a number of parallel plates in the manner characteristic of the family, the viscacha is distinguished from the other members of that group by having only three hind toes; while it is also the heaviest-built and largest member of the group, with smaller ears than the rest.