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THE AMEBIC AN NATURALIST [Vol. XLII 



the prairie region. This composite nature of the prairie 

 fauna may be seen by comparing the two lists above. 



3. There is a great difference in the extent to which the 

 forms of eastern North America push westward, or the 

 plains forms push eastward, into the prairie rcgionbefore 

 becoming modified or checked. Among the plains forms 

 the Arkansas kingbird (Tyrannus vertical is Say) and 

 the western hog-nosed snake (Heterodon nasicus B. & G.) 

 only extend into the western part of Iowa, Palo Alto 

 County, being apparently the most eastern authentic 

 locality recorded, 4 but the yellow-headed blackbird, Xan- 

 tliocephtdus x<nithoccphalus (Bonap.), and red-sided gar- 

 ter-snake, Thamnophis sirtalis parietalis (Say), extend 

 into Illinois. Again, the prairie hare (Lcpus cam pest ris 

 Bach.) and others only enter the western part of Iowa, 

 while the Franklin spermophile, Citellus franklini 

 (Sabine), penetrates eastward as far as Illinois, and the 

 thirteen-lined spermophile, Citellus tridecemlincatus 

 (Mitch.), extends to southeastern Michigan. 5 



Among the eastern forest forms, the Michigan mouse, 

 Peromyscus michiganensis (Aud. & Bach.), cotton-tail, 

 Sylvilagus floridamts mearnsi Allen, green snake, 

 Liopeltis vernalis (De Kay), bluebird, Sialia sialis 

 (Linn.), Baltimore oriole, Icterus galbula (Linn.), 

 orchard oriole, Icterus spurius (Linn.), bob-white, 

 Colinus virghiianus (Linn.), swamp sparrow, Melospiza 

 georgiana (Latr.), and others extend westward at least 

 as far as the western margin of the prairie, while the 

 Butler garter-snake, Thamnophis butleri (Cope), garter- 

 snake, Thamnophis sirtalis (L.), meadow lark, Sturnella 

 magna (L.), long-billed marsh wren, Telmatodytes 



