1903.J NATURAL SCIENCES OF PHILADELPHIA. 555 



100th meridian— a fact which suggests that species belonging to the 

 dry plains fauna may endure a small addition of moisture better than 

 those from a moist region can withstand drought. In a general way 

 it is also the region where elevations of less than 1,000 feet begin the 

 rise which reaches to between 3,000 and 5,000 on the plateau border- 

 ing the Pecos river. Its western boundary is probably the beginning 

 of what Cope terms "the first plateau." 



The geographical limits of Cope's Texan district are nowhere clearly 

 marked out by him, and his views as to its affinities were subject to 

 change. In 1875'^ he says the Sonoran ''occupies the lower valley 

 of the Rio Grande, and extends into Texas as far as the desert east of 

 the Pecos." East of this lay his Texan district, without defined limits 

 on the east and south, which he assigns to the Austroriparian subregion. 

 In 1880" he speaks of the Texan district as Sonoran, and appears to 

 consider it as beginning on the west at about 3,000 feet elevation, its 

 eastern boimdary running north and south near longitude 98° to 

 within twenty miles of San Antonio, from whence it stretched west to 

 the Rio Grande. In 1896^^ he returned without comment to his 

 earlier opinion and put the Texan district back in the Austroriparian. 



It may be possible at present to reach a definite opinion as to its 

 proper association. 



If the Texan district be regarded as beginning at the debatable zone 

 which has been indicated here, between 98° and 96° longitude, and 

 extending sovith to the Rio Grande and west to the Pecos river, it 

 is found that the elements composing its reptilian fauna are the 

 following : 



Sonoran: Chihuahuan ^9 



Central ^ 



— 47 



Texan (restricted) ^^ 



Austroriparian ^ 



Of wide range 



69 



As far as is now known several of its peculiar species, as Holbrookia 

 propinqua and Sceloporus thayeri, range slightly to the south and west, 

 outside of the boundaries here assigned to it, and may ultimately have 

 to be regarded as Chihuahuan, but in the main they appear to be con- 

 fined to these hmits. A guide to its western boundary may be found 



« Bull. U. S. Nat. Mus., No. 1, p. 68. 



" Bull. U. S. Nat. Mus., No. 17, p. 46. 



18 American Naturalist, November and December, 1896. 



