MAMMALIA. 93 
Colombia and Venezuela. The Geomyide, or Pocket-Gophers, are mainly Central 
American, Thomomys, Cratogeomys, and Geomys ranging into the Southern States of 
the Union, Platygeomys, Zygogeomys, etc., being Mexican, Orthogeomys extending 
from Mexico to Guatemala, and Macrogeomys occurring from Nicaragua to Panama. 
The paleontological history of the Voles (Microtine) before the Pleistocene is some- 
what meagre, but Microtus has been traced back to the Pliocene in Europe and to the 
Pleistocene in North America. Records of the Cricetine are more complete, and 
extend back to the Lower, Middle, and Upper Oligocene in Europe and North America, 
the existing Peromyscus dating in North America to the Miocene. In South America 
Cricetines appear to be of recent (Pleistocene) date. The Heteromyide and Geo- 
myide, closely related families, seem to have originated in North America, where 
they have been traced respectively to the Lower and Upper Oligocene. 
The palzontological evidence, therefore, suggests that the ancestors of the Central 
and South American Myomorpha came down from the north, This is no doubt true 
of the Geomyide and Heteromyide, but it is not so certain of all the Cricetines. The 
failure to discover fossil remains of small rodents such as these in mid-Tertiary 
(Upper Miocene) deposits in South America cannot be regarded as conclusive evidence 
of their absence there at the time the deposits were laid down; moreover, the 
occurrence of genera of this subfamily in Madagascar strongly suggests their existence 
in Africa at a sufficiently early date to permit their migration thence into South 
America by the land-connection which there are other grounds for thinking may have 
joined these two continents together. 
Suborder HYSTRICOMORPHA. 
Three families—the Octodontide, Dasyproctide, and Hystricidee—occur in Central 
America, but neither is peculiar to the country, though the Dasyproctide are found 
elsewhere only in South America and the West Indies. None of the Central 
American genera of these families enters the United States. 
Of the Dasyproctide, Dasyprocta (Agouti) ranges from Brazil through Panama to 
Mexico, probably Vera Cruz; <Agouti or Calogenys (Sooty Paca) also passes from 
Panama to Vera Cruz. ‘The Hystricide are represented by Coendou (= Synetheres), 
the common South American Tree-Porcupine, which extends from Bolivia and Brazil 
to the mountains of Mexico. In North America its place is taken by Lrethizon, 
distributed from Alaska and Canada to Arizona. One other genus of this family, 
Chetomys, occurs in South America only. ‘These arboreal Porcupines constitute the 
subfamily Erethizontine as opposed to the Hystricine or Ground-Porcupines of the 
Old World. ‘The Octodontide, generally distributed throughout South America, 
comprises a large number of genera, two of which, Loncheres and Proechimys, enter. 
Central America, the latter as far as Nicaragua, the former only to Panama. A third, 
Hoplomys, occurs in both these countries. 
