1903.] NATURAL SCIENCES OF PHILADELPHIA. 557 



Of Austroriparian genera these do not range into the Sonoran :™ 

 Alligator N. Farancia 



Anolis N. Rhadinia N. 



Ophisaurus Stilosoma 



Rhineura^. Cemophora 



Spilotes N, Seminatrix 



Abastor Liodytes 



I do not know of a single truly Sonoran genus which extends into 

 the Austroriparian which is not found in other subregions as well. 

 No strictly Austroriparian genus extends into the Chihuahuan, and 

 no genus is common to both areas which is not found elsewhere. So 

 far as genera are concerned, the only bond of unity between the two 

 halves of the Lower Austral ( = Austroriparian + Texan and Chihua- 

 huan of Cope) is that they alone in the Medicolumbian share Testudo, 

 Tantilla and Elaps, all of which are Neotropical. 



The Sonoran shares the following genera with the Central: Crota- 

 phytus, Holbrookia, Phrynosoma, Rhinochilus, Hypsiglena. 



It is clear, then, that the community in reptiles between the Sonoran 

 and the Austroriparian is much less than that between the Sonoran 

 and the Central; it is, in fact, limited to widely ranging genera. 



While it is not intended to go into the details of distribution through- 

 out North America, a few general observations must be stated, because 

 of their bearing upon the present question. 



The usual tests, such as differentiation, predominance in numbers, 

 and genetic relationship, used in determining centres of distribution, all 

 point among reptiles to the two southern centres in the Medicolumbian, 

 to which attention has been directed by Prof. C. C. Adams,^^ the loca- 

 tion of which corresponds to the southeastern Austroriparian and the 

 Chihuahuan. The former gave rise to nearly all the aquatic turtles. 

 It has supplied most of the widely ranging genera of snakes, some of 

 which, as Coluber, Zamenis and Tropidonotus, as well as E7nys among 

 turtles, are found also in the Eurasian portion of the Holarctic. Its 

 scarcity of lizards amounts to poverty. The reptiles of the Eastern 

 subregion are almost wholly derived from this southeastern centre; 

 no genus is peculiar to it, and but few species, especially east of the 

 Alleghenies. The differences between its eastern and western por- 

 tions are considerable, and in the Mississippi valley it shares a few 

 snakes with the Central, some of which are Texan and Chihuahuan. 

 Upon reptiles alone, the Eastern could hardly be maintained as a 

 subregion apart from the Austroriparian and of equal rank. 



'"> Those marked N. are Neotropical. 

 " Biol. Bull., Vol. Ill, p. 121 (1902) ■ 



