2 BULLETIN 86, UNITED STATES NATIONAL MUSEUM. 



Dr. L. B. Bishop, Mr. J. H. Fleming, and Mr. A. C. Bent. To the 

 authorities of these institutions, to the curators of their bird collec- 

 tions, and also to the gentlemen above mentioned, the writer wishes 

 here to express his thanks for their courtesies. Moreover, to Dr. 

 Charles W. Kichmond, of the United States National Museum, he is 

 as usual indebted for many kindnesses in connection with the present 

 investigation. The drawings of skulls on plates 1 and 2 were exe- 

 cuted by Miss E. G. Collette. 



For the names of colors Mr. Robert Ridgway's recently published 

 Color Standards and Nomenclature ^ has been adopted as the 

 standard. 



The numbering of the primaries is from the outermost, since this 

 is so much more convenient in descriptive ornithology, although,, of 

 course, indefensible from a morphological standpoint. 



Detailed synonymy has been omitted under the forms of both 

 Chordeiles virginianus and Chordciles acutipennis, because such 

 synonymy would be merely a duplication of that in Mr. Ridgway's 

 Birds of North and Middle America, part 6,^ which is soon to 

 appear. 



All measurements are in millimeters. The specimens used in the 

 average measurements under each subspecies, and with which com- 

 parisons are made, are taken just as far as possible from typical 

 specimens — that is, from examples best exhibiting the differential 

 characters. In the tables of detailed measurements all the birds used 

 in the diagnostic averages, as well as all other than adult birds, are 

 indicated b}^ reference marks. The various dimensions of which use 

 is made in the following pages have been taken as follows: 



Length of wing. — Measured in a straight line from the bend of 

 the closed wing to the end of the longest jorimary, with the primaries 

 in their natural position — that is, not straightened. 



Length of tail. — Taken with dividers from the point of insertion 

 of the middle rectrices to the tip of the longest. 



Exposed cidmen. — Measured in a straight line from the beginning 

 of the feathers on the culmen to the tip of the maxilla — that is, the 

 chord of the exposed culmen. 



Length of tarsus. — A straight line from the center of the heel joint 

 on the posterior side to the middle of the joint between the metatarsus 

 and the middle toe on the anterior side. 



Middle toe. — Measured along the upper side from the middle of 

 the joint between the metatarsus and the middle toe to the base of 

 the uncovered claw. 



■> Ridgway, Color Standards and Color Nomenclature, 1912 (=January 16, 1913), pp. 

 (1-4) ; i-iv ; 1-44; frontispiece; pis. 1-53; Washington, D. C. 



" Bull. U. S. Nat. Mus., No. 50, pt. 6, 1914. In this volume Mr. Ridgway has adopted 

 from my manuscript my conclusions regarding the subspecies of Chordeiles ; but he has 

 not seen the introductory portion of the present bulletin, pages 1-23. 



