DISTRIBUTIONAL PATTERNS OF VERTEBRATES 457 



(see Baker, 1947) or at most secondarily interbreed. A very wide 

 disjunction exists between an eastern treefrog, Hyla femoralis, 

 and the apparently related //. arenicolor of the west (Fig. 9) : H. 

 femoralis occurs on the coastal plain west to the embayment and 

 H. arenicolor ranges westward from trans-Pecos Texas, which means 

 there is a gap of some 700 miles between the ranges. 



Another disjunction involving the Mississippi Embayment and a 

 forest gap is that of the diamondback rattlesnakes. The eastern 

 diamondback (C adamanteus) is limited on the west by the embay- 

 ment; the western diamondback (C. atrox), with a wide range in 

 northern Mexico and the southwestern United States has its east- 

 ward distril)ution limited at the forest border. An apparently 

 isolated (relict) population of this species is known from the vicinity 

 of Tehuan tepee, Oaxaca, Mexico (Stebbins, 1954). A third isolate, 

 C. ruber, in southern California and Baja California, possibly stems 

 from a Pleistocene isolate in Baja California fsee Gloyd, 1940). 



The gopher turtles (Gopherus) also comprise three isolates (Fig. 

 10). The eastern species, G. polyphemus, ranges on the coastal plain 

 west to the vicinity of the Mississippi Embayment. Another, G. 

 berlandieri, occurs in northeastern Mexico and southern Texas and is 

 separated from the eastern species by a gap involving both grassland 

 and forest. The third disjunct. G. agassizi, ranges from northern 

 Sonora through western Arizona and southeastern California to 

 southern Nevada. 



The cricketfrogs (Acris) have overlapping ranges that involve the 

 Mississippi Embayment and the margin of the coastal plain. The 

 eastern coastal plain species, A. grylliis, is limited westward by the 

 embayment. The western species, A. crepitans, overlaps the range of 

 the eastern species just east of the embayment and along the Fall 

 Line (see Blair, 1958). The distributional relationships of these 

 frogs are comparable to those of the Peroniyscus leucopus group ex- 

 cept for the limitation of the eastern population by the embayment, 

 and they are interpreted similarly as the result of post- Pleistocene 

 spread from Floridian and IVlexican refuges. 



The distributional pattern of the Bujo americaniis group of toads 

 is a more complex variation of the same general pattern (Blair, 

 1958). The eastern coastal plain form, B. terrestris, is limited west- 

 ward by the Mississippi Embayment. A population to the north of 

 the Fall Line, B. americanus, is interpreted as having spread from a 



