ENVIRONMENTAL REACTIONS OF 

 PHRYNOSOMA^ 



A. 0. WEESE 



If General Disf rilixt'nni.^-The horned lizards, more 

 familiarly known as the "horned toads," of the south- 

 western portion of the United States and the northern 

 states of Mexico form a very distinct group of the family 

 Iguanidae. Unlike most other comparatively large rep- 

 tilian genera, this particular genus {Phrynosoma) is 

 limited to a very special environment, and it is only in 

 a region of relative aridity that these animals find a 

 favorable habitat. Within the limits set by the above 

 condition the specific habitats of the various species and 

 varieties of the genus vary greatly, ranging all the way 

 from the extreme aridity and great heat of Death Valley 

 in southern California {Phrynosoma calidiarum Cope) 

 to the comparative moisture and cold of the northern 

 Rockies {Phrynosoma douglassii Bell and varieties). 

 The species especially discussed in this paper are all 

 found in the Soiithwot. under varying environmental 

 conditions. 



Phrynosoma uajdcsiiDn, the specimens of which were 

 taken near Albuquerque, New Mexico, close to the lower 

 edge of the "mesa" or clinoplane region, at an altitude 

 of about 1,700 meters, is distributed throughout New 

 Mexico, and to a certain extent in the adjoining states, 

 wherever conditions are similar to those in the above 

 typical habitat. The rainfall here averages about 30 cm. 

 annually, while the yearly evaporation from a free water 

 surface is in the neighborhood of 200 cm. The soil is 



