DISTRIBUTIONAL PATTERNS OF VERTEBRATES 



455 



coastal plain to the Pearl River; the western crawfish frog (R. 

 areolata) is abundant on the coastal prairie in eastern Texas and 

 ranges northeastward in the grasslands but is limited in its eastward 

 distribution by the forest. 



6-OHuoi w«ii «oo 



Fig. 8. Present distributions of two closely related, allopatric species 

 of chorus frogs. 



Both forest and grassland occur in the hiatus between the eastern 

 and western populations of the indigo snake {Drymarchon corais). 

 The eastern population occurs on the coastal plain east of the 

 Mississippi Embayment; the western population ranges from the 

 area of Corpus Christi in Texas southward into northern South 

 America. The eastern black-headed snake (Tantilla coronata) occurs 

 east of the embayment, and its western counterpart, T. gracilis, 

 lives west of the forest border. Eastern and western populations of 

 Amphiuma, distinguished by a difference in the number of toes, 

 meet at the Mississippi Embayment where they may act as species 



