NEOTROPICAL MAMMALIA. 75 



principally to the Alpine slopes of the Andean chain, in Peru. Bo- 

 livia, and Chili, but also (Lagostomus) inhabiting the lowland plains 

 of the Argentine Republic and Uruguay ; the Caviidse, the cavies 

 (Cavia) and agouties (Dasyprocta), a family of rodents to which the 

 guinea-pig belongs, and whose members range from Mexico to be- 

 yond the forty-eighth parallel of south latitude ; the Bradypodidse, 

 or sloths, confined exclusively to the forest region ; the Dasypodidse, 

 or armadillos, which are found throughout almost the entire region, 

 with one species (Dasypus peba) penetrating as far north as Texas ; 

 and the Myrmecophagidae, or ant-eaters. The Cebidae, or South 

 American monkeys proper, constitute a very distinct group of quad- 

 rumanous animals (Platyrhina), distinguished from the monkeys of 

 the Old World (Catarhina) by several very prominent characters, 

 such as the broad nasal septum (whence the designation of flat- 

 nosed monkeys, Platyrhina), the absence of ischial callosities and 

 cheek-pouches, and the presence of an additional premolar tooth on 



18 

 each side of each jaw, making the total dental formula -— = 30, in- 



18 



stead of — = 32, as we find it in the Old World apes and man. In 

 IG 



the smaller group of the marmosets and lion-monkeys (Jacchus, Mi- 

 das) we find the same number of teeth as in the catarhines, but the re- 

 lation of the premolars to molars is reversed, i. e., they are arranged 



3—3 2—2 . ^ , 2—2 



according to the formula pm., m. ; instead of pm. ~ — ^, 



3 — 3 2 — 2 u — a 



3 3 



m. . In none of the American monkeys is the thumb com- 



3—3 



pletely opposable to the other fingers, an important distinguishing 

 cliaracter; and scarcely less important is the presence, in most cases, 

 of a prehensile tail, which, as such, is developed only in this group 

 of the Quadrumana. The range of the American monkeys, in 

 marked contrast to that of the African, is limited virtually to the 

 forest region, in which alone they find their proper sustenance. 

 Their most southern extension appears to be about the thirtieth 

 parallel of south latitude, and their most northern, the southern 

 portions of Mexico. No representatives of the order are met with 

 in any of the West India Islands, which are also wanting in all Car- 

 nivora and Edentata. The Platyrhina comprise, among other groups, 

 the well-known howling-monkeys (Mycetes), spider-monkeys (Ate- 



