46 Nelson — North American Mainland Myiarchus. 



Breeding range.— CozMva^l Island, Peninsula of Yucatan, and coast 

 region of Campeche and Tabasco, to the Grijalva River. Not migra- 

 tory. 



Zonal distribution. — Arid and Semi-arid Tropical. 



Subspecijic characters. — Most like olivascens, but upper parts darker; 

 back more greenish, and tail coverts, tail and wings darker and more 

 strongly edged with russet or reddish cinnamon; bill averages broader. 



Description of fresh plumage. — Crown dark brownish more or less 

 heavily washed with olive; back slightly greenish olive; upper tail 

 coverts broccoli brown edged and sometimes shaded with russet or red- 

 dish cinnamon; tail feathers edged externally like upper coverts; all 

 but two outer primaries narrowly and secondaries more broadly edged 

 externally with russet like tail; wing coverts broadly tipped with dark 

 Isabella brown; chin, throat and breast ashy gray; inner webs of tail 

 feathers usually plain dusky, but occasional individuals have narrow 

 buffy edges along inner borders of inner feathers. Worn specimens lose 

 nearly or quite all the greenish shade on back and the rusty edgings to 

 wings and tail. 



Measurements of specimens from Cozumel Island: — Averages of five 

 adult males: Wing, 81 (79-83); tail, 77.7 (74-82); culmen, 17 (16.5-17.5); 

 tarsus, 20.2 (20-20.5). 



Averages of two adult females: Wing, 75 (75); tail, 69 (69); culmen, 

 16 (16); tarsus, 18.5 (18-19). 



Mainland specimens average about the same. 



Oeneral notes. — A series of 33 specimens in the Biological Survey and 

 National Museum collections, from various localities covering the range 

 given above, show conclusively that the birds from this region and 

 especially from Yucatan which were formerly referred to lawrencei and 

 later to olivascens are identical with platyrhynchus, described by Mr. 

 Ridgway from Cozumel Island. This form occupies the comparatively 

 arid region of northern Yucatan and the adjacent coastal area to the 

 exclusion of true laiorencei, which belongs to the more humid interior 

 bordering the Cordillera. 



Among the entire series only a single specimen, and it is from Cozu- 

 mel Island, has a buffy border to inner webs of inner tail feathers. 



In view of their wide separation, necessarily distinct origin and dif- 

 ferent environment, the close general similarity between platyrhynchus 

 and olivascens is interesting. It is another of the many cases in which 

 great similarity exists between widely separated forms of a species with 

 one or more decidedly more differentiated forms occupying the interven- 

 ing area. The greater humidity of the habitat of platyrhynchus accounts 

 for the somewhat darker colors of this form in comparison with the paler 

 and grayer colors of olivascens. 



