120 GLIRES. 
Order VII. GLIRES. 
Suborder I. SIMPLICIDENTATA. 
In this the dominant suborder of Rodents* the aspect of the Central-American fauna 
is of a very mixed character, Neotropical types preponderating in some groups and 
Nearctic in others. 
In the first section, the Sciuromorpha, this is well illustrated in the Sciuride, where 
we find that our representatives of Sciwrus are mostly southern forms, that Tamias is 
absent, and that Cynomys just crosses our northern boundary ; while, on the other hand, 
such characteristically northern genera as Sciwropterus and Spermophilus are well esta- 
blished in Mexico. The family Haplodontide is not represented ; but the Beaver, sole 
surviving member of the Castoride, is found along the Nearctic frontier. 
Of the Myomorpha the Muride are decidedly Nearctic. Passing over the introduced 
species of Mus, we find that the known Central-American species of Hesperomys all 
belong to the North-American subgenera or sections of that large genus, except a few 
which appear to be peculiar to the subregion. ‘The allied Nearctic genera Ochetodon, 
Sigmodon, Neotoma, and Arvicola are all represented; while the Neotropical forms 
Drymomys, Holochilus, and Reithrodon seem to be totally absent. The Geo- 
myide probably find their greatest development in Central America, but are also 
spread over most of North America, whereas only one or two forms are known to 
extend south of the Isthmus of Panama. The Nearctic family Zapodide is not 
represented. 
In the Hystricomorpha we have a characteristic Neotropical type in the Dasyproc- 
tide, and the only Central-American species of Hystricide belongs to the South-American 
genus Synetheres. On the other hand, the great groups which lend such a peculiar 
character to the Neotropical fauna are either poorly represented or wanting; the large 
family of Octodontide has only a single form, Myopotamus, and the Chinchillide, 
Dinomyide, and Caviide are conspicuous by their absence. 
On balancing the affinities of the whole order, the Nearctic types of Rodents must 
be considered to preponderate in numbers in Central America, notwithstanding the 
presence of some very characteristic Neotropical forms. | 
* The arrangement of the Rodents here adopted is essentially that proposed in my paper “ On the Classifi- 
cation of the Order Glires” (P. Z. 8. 1876, pp. 61-98, pl. iv.). 
