SQUIRRELS OF MEXICO AND CENTRAL AMERICA 1 7 



Thomas responded very handsomely, devoting considerable 

 time to the comparison and returning the specimens with an- 

 notations which have made it possible to state authoritatively 

 exactly what species were represented. 



Within the area covered by the present paper Gray, in 1867, 

 recognized 18 species and varieties, Allen in 1877, 6 species and 

 3 varieties, and Alston in 1880, only 7 species. As a result of 

 the accumulation of modern material a number of new forms 

 have been described by various authors, and in the present re- 

 vision 43 species and subspecies are recognized. ^ In order to 

 understand the relationships of these numerous species it is 

 necessary to have a general idea of their surroundings and the 

 topographic features of the region they inhabit. 



PHYSIOGRAPHY OF THE REGION. 



Mexico and Central America present great contrasts of topo- 

 graphic and climatic conditions, accompanied by wonderful 

 variety of animal and vegetable life. The mountain ranges 

 fronting the two coasts of Mexico, and uniting near the Isthmus 

 of Tehuantepec, are of great magnitude and form the outer 

 border or rim of the broad interior tableland. Southward the 

 continental area is narrow and the elevated interior correspond- 

 ingly reduced. One of the most characteristic features is the 

 comparatively level lowland skirting both coasts between the 

 sea and the base of the mountains. This coastal plain is some- 

 times 100 miles wide and often broken by hills or low ranges 

 of mountains which extend out from the main interior ranges as 

 short spurs, or rise island-like in the midst of the plain. The 

 hills reach the sea at many points but usually the country near 

 shore is level. The northern parts of the coastal plains, except 

 a very narrow belt along the east shore of Mexico, lie within 

 the Lower Sonoran zone and in passing southward, gradually 

 merge into the Arid Tropical. On the west side the arid char- 

 acter of the coast belt extends at least to Costa Rica, but on the 

 east coast it gives way in the southern half of Vera Cruz to the 

 Humid Tropical zone which, with the exception of the arid 



1 Several South American species have been reported from the Isthmus of 

 Panama but in the absence of any material have necessarily been omitted. 



