92 ORIGIN, ETC., OF THE FAUNA. 
known in South America, unless of quite recent date, it seems clear that Central 
America received this prominent element of its fauna from North America. : 
Beavers also seem to have originated in North America, where they date back to 
the Middle Oligocene, but Castor itself appeared in Europe in the Pliocene and in 
North America in the Pleistocene. 
Suborder MYOMORPHA. 
- The Central American Myomorphs belong to the three families Muride, Hetero- 
myide, and Geomyide, the two last-named being restricted to America. 
The genera of Muride belong to the three subfamilies Microtine, Neotomine, and 
Cricetine. Ofthe Microtine, Microtus (s. s.) extends over the greater part of Europe, 
Asia, and North America, and enters Mexico (Puebla, Jalisco, Oaxaca). Two sub- 
genera are known only from Central America, namely, Orthriomys from Oaxaca and 
Herpetomys from Guatemala, both at high altitudes. The other genus Pitymys has a 
singular distribution, ranging in the Old World from South and Central Europe to 
Asia Minor (the Mediterranean Area), and through the Eastern States of North 
America to Mexico (Vera Cruz). 
Of the Neotomine, Xenomys, Hodomys, Nelsonia, and Teanopus are confined to 
Mexico, while Neotoma, represented by a host of species, overlaps them to the north 
and south with a range from the Southern States of North America to Guatemala. 
The Cricetinz have an extraordinarily wide distribution, namely Africa, including 
Madagascar, Europe, Asia, and practically the whole of America. The Central 
American genera are too numerous to mention in detail. Some occur in North and 
Central America, others in Central America alone, others are common to Central and 
South America, while others extend over the three areas mentioned. To the last 
category belong Reithrodontomys, Oryzomys, and Sigmodon, ranging from the Southern 
States of the Union to Ecuador, Chili, and Peru, respectively. As examples of the 
first group may be cited Peromyscus (Hesperomys) distributed from Labrador to 
Panama, and Baiomys and Onychomys from the Southern States to Guatemala and 
Panama. Peculiar to Central America are Nyctomys and Ototylomys, ranging from 
Mexico to Panama and Guatemala, and Sigmodontomys known only from Costa Rica, 
while Akodon, extending all over South America from Costa Rica, and Nectomys, 
occurring in Nicaragua, Ecuador, and Colombia, are representatives of genera common 
to Central and South America. 
The genera of Heteromyide belong to two subfamilies, the Heteromyine and 
Dipodomyine. Of the latter, Perodipus and Dipodomys (Jerboa-Mice) are restricted 
to the South and South-western States of North America and to Mexico (Zacatecas, 
Vera Cruz, etc.). Of the former, Heteromys, Perognathus, Liomys, and Chetodipus 
occur in the Southern United States and Mexico, the first passing southwards into 
