The three remaining families of the Hystricoidea, of which one is African while the other two are chiefly South American, are very closely allied and often brigaded in a single family group. In the Capromyidae, which includes only the South American and West Indian hutias, the South American coypu and the African cane-rats, the tympanic bulla of the skull is hollow, the par-occipital process straight, the lachrymal small, and the cheekteeth rooted, with deep enamel-folds; the first front toe Leing occasionally absent.
Of the few living representatives of the group, the genus Myocastor (or Myopotamus) is represented only by the South American coypu, M.
coypu, which is aquatic in its habits, and measures about 2 ft.
Is a rodent known in natural history as the coypu, about half the size of a beaver, and when unhaired has not more than half, generally less, the depth of fur, which is also not so close.
The rodents are the most numerously represented order, which includes the coypu or nutria (Myopotamus coypus), the chinchilla (Chinchilla laniger), the tuco-tuco (Ctenomys brasiliensis), a rabbit, and 12 species of mice - in all some 12 genera and 25 species.