458. ZOOLOGY. 
limbs, and with ears likewise short. The feet are naked beneath ; the molar 
teeth nearly of equal size, each molar with two principal lobes. ‘To this 
genus belongs the domesticated Guinea pig, Cavia cobaya (pl. 113, fig. 2 a); 
it inhabits the banks of the Rio de la Plata, and extends northwards into 
Paraguay, Bolivia, and Brazil. — 
Fossil species of this genus were found in the caverns of Brazil; it is 
interesting to know that another species existed in seoabte S durig the 
tertiary epoch. 
Some species of cavies are grouped under the sub- “generic name of 
Cerodon or Kerodon, to which three South American apeeres are referred, 
from the diluvial pened: 
The genus Hydrocherus includes cavies with the upper incisor teeth 
having a broad and shallow groove in front; the molars unequal in size ; 
the feet short and broad, and semi-palmated, the toes terminating in broad 
and depressed nails; the ears small. The capybara (4. capybara) is the 
only living species known; a second, but fossil and extinct, has been disco- 
vered in the caverns of Brazil. 
Sub-fam. 2. Chinchillina (Lagostomide of some authors, both names 
being indifferently used) are those hystricines with rootless molar teeth, 
having parallel, or nearly parallel transverse plates of enamel; the series of 
molars on either side of each jaw converging in front; the tail is long or of 
moderate length, recurved and bushy; clavicles perfect and slender. They 
inhabit the mountains of Peru and Chili, and one species occurs in the 
plains of La Plata. 
The genus Lagostomus is composed of but one species (the viscacha), 
that which inhabits the plains of La Plata. It is burrowing in its habits, 
easily distinguished from the other Chinchillina by the reduced number of 
the toes, three in number, to its hind feet, and the comparatively long, com- 
pressed, and sharply pointed nails, with which they are provided. The fore 
feet have four toes, armed with rather short, arched, and pointed nails. 
The upper lip has a vertical groove; the snout is broad and expanded. 
A fossil species of this genus occurs in the Brazilian caverns. 
The genus Lagidiwm is characterized by long ears, a tail long and bushy, 
the tarsi entirely naked beneath, with four toes to the fore feet, the nails of 
the toes short. Two species belong to this section. 
The genus Chinchilla differs from the preceding only in being provided 
with large and rounded ears, five toes to the fore feet and four behind. The 
name of Hriomys is sometimes given to this genus; Hriomys laniger, or 
Chinchilla lanigera (pl. 118, fig. 1). As belonging to this group, and more 
particularly related to Lagostomus, we must insert here the genus Megamys, 
one of the largest known, although extinct, rodents. It contains but one 
species, from Patagonia. 
Sub-jam. 3. Octodontina are hystricines with rootless molar teeth, having 
but a single indenting fold of enamel on either side, or rarely with an extra 
fold on the inner side of the molars of the lower jaw; zygomatic arch with 
an angular process on the lower edge; the hind feet provided with five 
toes; the fore feet likewise with five toes, or sometimes with four. The 
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