488 RODENTIA 
about 10 inches in length, the tail somewhat exceeding 5 inches, 
and the ears very large. Its fur is greatly valued on account of 
its extreme softness and delicate gray colour. 
Lagidium+ and Lagostomus.°—Lagidium has four digits in both 
fore and hind feet, and Lagostomus three only in the hind feet, 
and the auditory bulle are much smaller than in the preceding 
genus. Lagidivim has the same distribution as Chinchilla - while 
Lagestomus, as represented by the Viscacha (L. trichodactilus), is 
found in the Pampas from the Uruguay River to the Rio Negro. 
The Viscachas live in burrows, generally in large numbers, and are 
nocturnal in their habits. Remains referable to the existing species, 
as well as others which appear to belong to extinct forms, occur in 
the Pleistocene deposits of South America. 
Extinct Genera. —Several Rodents from the South American 
Tertiaries more or less closely allied to Lagastomus have been 
described by Dr. Ameghino under the names of Prolagostoimus. 
Plislagostemus, ete. The huge Megamys (Potamarchus), from the 
infra-Pampean deposits of Parana and Patagonia, is referred to this 
family, and has dimensions approximating to those of an Ox. 
Other fossil genera have received the names of Eyillema and Tetra- 
stylus. 
Family CASTOROIDID. 
Castoroides.°—The large Beaver-like Rodent with the dimensions 
of a Bear from the Pleistocene of the United States described 
under this name is regarded by Dr. Coues as the type of a family. 
Its dentition is nearest to that of Chinchilla and Hydrocherus, but 
some of the cranial characters are like those of the Custoride. The 
genera Amblyrhiza and Loxomilus, from the Pleistocene of the 
Antilles, appear to be allied types. 
Family DASYPROCTIDE. 
Terrestrial forms with subequal limbs, hoof-like claws, short or 
obsolete tail, and rudimentary clavicles. Mandibular masseteric 
ridge obsolete ; palate broad; incisors long: molars semi - rooted, 
with external and internal enamel-folds. Neotropical region. 
Dasyprocta.s—Includes several slender-limbed species, with three 
hind toes, commonly called Agoutis, inhabiting Central and South 
America, one (D. cristata) extending into the West-Indian Islands. 
Numerous fossil remains of this genus occur in the cavern-deposits 
of Brazil. 
1 Meyer, Vora deta Ac. Cars. Lrop.-Car. vol. xvi. p- 576 (1833). 
? Brooks, Trans. Linn. Sor. yol. xvi. p. 102 (1828. 
® Foster, Sccond Rep. Geol. of Ohio, p- $1 (183s . 
* Illiger, Prodromus Sust. Memin. p. 93 (1811). 
