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 cohaya (jyl. 113, j^y. 2 a); 

 it inhabits the banks of the Eio de la Plata, and extends northwards into 

 Paraguay, Bolivia, and Brazil. 



Fossil S2:^ecies of this genus were found in the caverns of Brazil ; it is 

 interesting to know that another species existed in Europe during the 

 tertiary epoch. 



Some species of cavies are grouped under the sub-generic name of 

 Cerodon or Kerodon^ to which three South American sjjecies are referred, 

 from the dihivial period. 



The genus Hydrochoerus 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 {H. capyhara) is the 

 only living species known ; a second, but fossil and extinct, has been disco- 

 vered in the caverns of Brazil. 



Sah-fam. 2. ChincliiUina {Lagostomidce 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 

 mohirs 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 sjDCcies occurs in the 

 j)lains of La Plata. 



The genus Layostomus 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 Lagidium 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 Eriomys is sometimes given to this genus ; Eriomys laniger, or 

 Chinchilla lanigera (jpl. 113, /i^, 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-fam. 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 wnth 

 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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