1904,] NATURAL SCIENCES OF PHILADELPHIA. 465 



zone of various authors in other groups. If a transition zone existed 

 in the Nearctic for reptiles it would follow meridians of longitude west 

 of the Mississippi river, instead of extending from c ast to west. 



I ha\'e stated elsewhere* that the present distribution of Nearctic 

 reptiles points to the two post-glacial centres indicated by Prof. C. C. 

 Adams/ one in the southeastern Austroriparian, the other southwest- 

 ern, in the Sonoran. Some space must now be given to the evidence. 



In the same paper it was shown that m the southern Nearctic the 

 eastern and western reptilian faunas are so distinct that the community 

 between them is limited to a few species belonging to widely ranging 

 genera, and that the zone of change between them lies approximately 

 between the 96° and 98° meridians of longitude in Texas. In a 

 general way this zone may be carried north along those meridians, to 

 mark the separation between the whole Atlantic and the whole Sonoran 

 faunas, widening, however, toward the north in following up the rivers 

 of the Mississippi drainage. 



The division is in some measure obscured by the presence in the 

 lowlands of the Mississippi and its tributaries of a few species, chiefly 

 serpents, originally derived from both faunas and occupying the 

 contiguous portions of their respective areas. 



The proposition that original post-glacial centres must have lain 

 far to the south, is more logically necessary in the case of reptiles than 

 with other vertebrates, and follows from their dependence as a class 

 upon warmth. If further evidence were needed, it would be found in 

 their great numerical superiority, both in species and iiidividuals, in the 

 south. The whole extent of North America reaching from the border 

 of the great plains eastward to the Atlantic coast, and from the Gulf 

 of Mexico to the northern limit of reptiles, which, following Agassiz, 

 will here be known as the Atlantic subregion, to avoid confusion 

 with Its Eastern district, contains 113 species and recognizable sub- 

 species in all orders of reptiles. Of these 56 are confined to some por- 

 tion of the Austroriparian district, and 15 are exclusively Eastern 

 with 42 common to both, practically all of which are southern intru- 

 sions to varying distances into the Eastern. In the western Nearctic 

 the distribution of species is vastly more complex as to its details 

 on account of the great variety of surrounding conditions in contiguous 

 areas produced by rapid changes in altitude, and is not yet fully known 

 but the excess of southern species is even greater, for assuming that 

 5'l^^^^;i;^^^n^s_dnasi^^ from the western border of the 



I Proc. Acad. Nat. Sci. of Phila., 1903, p. 551 

 Biological Bulletin, Vol. Ill, p. 121 (1902). 

 •SO 



