20 Proceedings of the Biological Society of Washington. 



ourselves that our efforts to disentangle its complicated synonymy have 

 been entirely successful. Good series of specimens are absolutely necessary 

 in a case of this kind, and it is easy to see how much of this confusion has 

 arisen when we remember the amount and kind of material with which 

 authors generally have had to deal. With an unusually large and finely 

 prepared series of specimens for study we are satisfied that the tendency 

 toward the reduction in the number of specific types has gone too far in 

 this genus, if not also in other genera of the Tyrannidae. If M. ridgwayi is 

 specifically distinct, so also is M. mastacalis from M. barbatus, which can 

 invariably be distinguished one from the other by characters fully as good 

 as those which apply to M. ridgwayi. M. atricaudus, as shown beyond, is 

 not so closely related to M. barbatus as has generally been supposed, and 

 very probably their respective ranges overlap. M. semiflavus, an extreme 

 development of M. barbatus, inhabits a region adjacent to that occupied by 

 a race of M. atricaudus, with no sign of intergradation. M. atricaudus 

 and M. sulphureipygius aureatus are known to occur together, and Dr. 

 Chapman has lately shown that M. villosus, as the Subtropical Zone 

 representative of the latter, must be distinct specifically. In short, there 

 is every reason to believe that the forms whose ranges adjoin are fully 

 as distinct from each other as those whose ranges are actually known to 

 overlap. In discussing the case of Ewpsychortyx (Auk, XXXVII, 1920, 

 213), we have already had occasion to animadvert upon this matter, and 

 we here reiterate our belief that the fact of geographical collocation ought 

 not to constitute the sole (or even the principal) criterion for the recognition 

 of species as distinguished from subspecies. Take the case of the genus 

 Empidonax, for example — a storm-center for many years among American 

 ornithologists — whose members are now known to be distinct but closely 

 allied species, distinguished by slight but constant characters, and different 

 in life, nesting, etc. Caution is certainly necessary when dealing with 

 such a family as the Tyrannidae, in which slight differences often have 

 great value. We are convinced that in the case of Myiobius we have to do, 

 as in Empidonax, with a group of closely related but in the main distinct 

 species, and we venture to predict that when their life-histories come to be 

 as well known as are those of the North American Empidonaces similar 

 differences in their notes, haunts, nests, and eggs will come to light. 



Viewing the group as a whole, therefore, and essaying a consistent treat- 

 ment on the above lines, we would recognize seven species and four addi- 

 tional subspecies, ranging from southeastern Mexico to southern Brazil, 

 mainly in the Tropical Zone. The key which follows, although not strictly 

 dichotomous, is believed to indicate approximately their real relationships 

 and place them in their natural order as nearly as may be. 



Key to the Species and Subspecies of Myiobius. 

 A. Wing longer than tail; tail nearly or quite even; under parts green- 

 ish or yellowish, with little or no buffy. 

 a. No ochraceous color on breast and sides. 



b. Rump barium yellow; under parts Martins yellow, more or 

 less shaded anteriorly and laterally with old gold. 



Myiobius mastacalis. 



