CLASS I. MAMMALIA: ORDER 7. RODENTIA. 
391 
THE ROCK CAYY. 
of Buffoii ; the FerTcel-maus of the Germans, It is sometimes called Coui-coui, from its cry. There 
is both a wild and a domestic breed. In a wild state, it has the incisors white ; fur long, and 
somewhat coarse, and on the upper parts of the sides and body distinctly penciled with black and 
dirty yellow; chest gray-brown, throat and abdomen pale, dirty yellow, or sometimes brown-gray. 
It is of the size of a large rat, its legs very short, its neck short, and its body very clumsy. It has 
no tail. It inhabits the banks of the Rio de la Plata, and extends northward into Paraguay, 
Bolivia, and Brazil. It is known by the name of Aperea, and it is exceedingly common in the 
neighborhood of several towns on the banks of the Rio de la Plata, sometimes frequenting the 
sand hillocks, or the hedge-rows formed of the Agave and Opuntia ; but marshy places, covered 
with aquatic plants, appear to be preferred. 
Where the soil is dry, it makes a burrow, but otherwise lies concealed amid the herbage. It 
generally comes out to feed in the evening, and if the day be gloomy, it will likewise make its 
appearance in the morning. Mr. Bridges states that this animal in Bolivia is confined to the low 
lands, and in this respect differs from the Cavia Boliviensis^ which is always found at a consider- 
able elevation. It is not uncommon in fields in the neighborhood of Chuquisaca and Cochab- 
amba, and takes shelter among the loose stones of the walls by which the fields are inclosed. In 
Paraguay it generally frequents moist situations, and near the borders of forests, but it never 
occurs either in the forests or in the open fields. It lives in little societies of fi-om six to fifteen 
individuals, in the impenetrable groves of bromelias, and its presence is detected by numerous 
little beaten paths which it forms among these plants. It feeds early in the morning and after 
sunset in the evening, but never strays far from its home. It is stated that it breeds but once in 
the year, and then has but one or two young. 
The Domestic Guinea-Pig, or Cobaya, or x\ncema, the Mus porcellus of Linnseus, is probably 
the descendant of the wild species just described, though this some naturalists doubt, as the 
Aperea is said to breed but once a year, and to produce only one or two at a time, while the do- 
mestic animal is prolific as a rabbit ; the color, also, of the tame variety is greatly diversified. In 
