93 



Tropidonotus validus celaeno. 

 Charina bottae. 

 Lichanura trivirgata. 

 Licbamira myriolepis. 

 Licbsniira roseofusca. 



LaccrtiUa, 



Pbyllodactylus uuctus. 

 Pbyllodactylus xauti. 

 Cnomidopborus maxiinus. 

 Verticaria byperytbra. 

 Callisaurus dracontoides. 

 (Jta tbalassiiia. 

 Uta iiigricauda. 

 Sceloporus claikii zosteromus. 

 Pbrynosouui coronatum. 

 Cyclura beiiiilopba. 



XI.— RELATION OF DISTRIBUTION TO PHYSICAL CAUSES. 



The first observation witb regard to tbe Batracbian and Eepiilian 

 fauna of Nortb America is tbe usual one, viz, that tbe number of spe- 

 cific and generic types exhibits a rapid increase as we approach tho 

 tropics. Of the area inhabited by these forms of animals, less than one- 

 fonrtli is included in the three Southern regions — the Austroriparian, 

 the Sonoran, and the Lower Californian ; yet these contain more than 

 half of the entire number of species, and all but eight of the genera are 

 fouud in them. Of this number, forty-two genera, or one-third of the 

 total, is confined to within their boundaries. It is a truism directly 

 resulting from tbe very small production of animal heat by these ani- 

 mals, that temperature, and therefore latitude, has tbe greatest influ- 

 ence on their life and distribution. This is exhibited in other ways than 

 iu multiplication of forms. It is well known, that although plainly-col- 

 ored reptiles arc not wanting in tbe tropics, brilliantly-colored species 

 are much more abundant there than in temperate regions. Although 

 the Kegnum Xearcticum does not extend into the tropics, its south- 

 ern districts are the habitat of most of the species characterized 

 by bright colors. This is most instructively seen in species having a 

 wide range. Such is the case with the southern subspecies of Desmatog- 

 nathus among salamanders, and Ilyla among frogs. So with snakes of the 

 genera CrotaUis, Caudisona^ Ophiholus, Bascanium, and Euiaenia. It is 



