140 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



seems to be geuerally conceded, however, that Professor Baird was right 

 iu this determination ; therefore, as Dr. Coues surmises (Pr. Ac. Nat. Sci. 

 Phila. 1872, p. 68), Tyrannula mexicana, Kaup, and T. cooperi, Kaup, 

 must have been based upon variations of one species — the one under con- 

 sideration ! 



In attempting to determine to which of the two forms of the species Dr. 

 Merrill's specimens belonged, I found it necessary to carefully examine all 

 the material available. This consisted of twenty-four specimens, belong- 

 ing chiefly to the collection of the National Museum. After taking care- 

 ful measurements of every specimen, and submitting all to the closest 

 scrutiny and comparison, I found myself forced to a conclusion different 

 from that reached by Dr. Coues (see Bull. U. S. Geol, and Geog. Surv. 

 Terr. IV, No. 1, pp. 32, 33), in whose opinion regarding the matter I had 

 previously coincided. I now find, that (1) while extreme examples of 

 var. cooperi are astonishingly different from erythrocercus proper, such 

 individuals form a very small proportion of the whole, an<l are chiefly 

 from Western and Southwestern Mexico, where several other species of 

 birds, notably Fyranga cestiva (var. cooperi^ Ridgw.), attain the same 

 great development of the bill (and, in foct, all the measurements) ; that 

 (2) nearly all specimens from Mexico and Guatemala should be referred 

 to cooperi^ examples referable to erythrocercus on account of dimensions 

 and shades of color being comparatively rare. Besides averaging larger 

 than var. erythrocercus, var. cooperi has usually a grayer cast of plum- 

 age, in this respect corresponding to the M» xican race of Tyrannus 

 melancholicus (var. couchi, Baird), Myiarchus lawrencii, and numerous 

 other birds of similar geographical distribution. The only satisfac- 

 tory test, however, which I have been able to apply in determining to 

 which race doubtful specimens should be referred is that of size, as 

 follows : — 



Var. ERYTHROCERCUS. — Wing, 3.40-3.95 (average, 3.77) ; tail, 3.60- 

 4.00 (average, 3.82); bill, from nostril, .55-68 (average, .01); tarsus, 

 .85-.88 (average, .86).* Hab. — Eastern Tropical America, from Para- 

 guay to Southern Mexico, but chiefly southward of the latter country. 



Var. COOPERI. — Wing, 3.85-4.45 (average, 4.15); tail, 4.00-4.00 (aver- 

 age, 4.25) ; bill, from nostril, .60-.82 (average, .09) ; tarsus, .88-.95 (av- 



* Six specimens, as follows :— 



L(M-;ilitv. 



Date. 



"Brazil" 



Paraguay 



Costa Rica 



Merida, Yucatan 



.J,. . . do 



Sta. Eflgeuia, Tehuantopoc 



Aug. — , 1859 

 May — , 1859 



May 29,1805 

 Apr. 9, 1865 

 Dec. 18, 1868 



3.65 

 3.75 

 3.90 

 3.95 

 3.4) 

 3.95 



3.60 

 3.80 

 3.80 

 3 95 

 3.75 

 4.00 



.59 

 .60 

 .62 

 .63 

 . 55 

 .65 



