95 



Another peculiarity of the Sonoran regiou, and which it shares 

 with a part of Mexico, is the predominance of snakes which possess an 

 extraordinary development of the rostral shield either forward or out. 

 ward. This has also been observed by Professor Jan, who referred 

 such genera to a group he termed the ProhUtorMmdae, but which has 

 not sufficient definition to be retained in the system. Of ten genera of 

 suakes in the Nearctic region which possess the character, nine are 

 found in the Sonoran subregion, five are peculiar to it, and it shares 

 two with the Lower Californian subregion only. One of the latter 

 (Phimothyra) is closely imitated by a genus [Lytorhynchus) which occurs 

 on the borders of the African Sahara. The Heterodon of the Eastern 

 States, though not confined to the sandy coast-regions, greatly abounds 

 there ; and the South American species skip the forest-covered Amazon 

 Valley and reappear on the plains of the Paraguay and Parana. As 

 the Sonoran region embraces a number of desert areas, it is alto- 

 gether probable that the peculiar forms in question have a direct rela- 

 tion to the removing of dry earth and sand, in the search for concealment 

 and food. A modification of foot-structure, supposed to have relation 

 to the same end, is seen in the movable spines on the outer side of the 

 foot in the genus TTma, a character exhibited in higher perfection in the 

 South African genus Ptenopus. * 



The abundance of Bufones is doubtless due in j)art to their adapta- 

 tion to life in dry regions. They are mostly furnished with tarsal bones 

 especially developed for excavating purposes. 



* Proc. Acad. Phila., 1868, p. 321. 



