BIRDS OF NORTH AND MIDDLE AMERICA. 



493 



KEY TO THE SPECIElS OF GEOSPIZA. 



(Based on adult or nearly adult males only.^) 



a. Bill relatively short and stout, the transverse basal width of the mandible 

 (measured across interramal space) nearly or quite equal to, sometimes greater 

 than, length of gonys. {Geospiza). 

 h. Culmen not less than 19.05 (usually mucli nnjre) ; wing more than 76.20 (or else 

 culmen more than 20.32) . 

 c. Bill shorter and thicker (depth at base decidedly greater than length <if max- 

 illa from nostril) . 

 (/. Wing more than 90.17; culmen 26.42 or more. (Charles Island; extinct?) 



Geospiza magnirostris (p. 495) 



[Footnote— Continued.] 



Among the true Geospizw, nos. 1 to 13, inclusive, constitute a fairly gradual transi- 

 tion between extremes, which becomes much more evident when the average meas- 

 urements of specimens of each form from different islands are compared, when many 

 api^arent gaps are bridged over. The forms numbered 14 to 17, inclusive, do not 

 belong to the transitional series, but are "offshoots" from some of the forms com- 

 posing the latter. 



^ Owing to the fact that coloration affords little or no assistance in determining the 

 various forms of this genus, a satisfactory key i.s hardly possible. The one here given 

 is the best that I am able to devise under the circumstances, and is offered with full 

 consciousness of its inadequacy, but with the hope that it may be of some assistance. 

 Nearly all measurements of the more nearly related forms inosculate, so that it is 

 seldom that positive characters can be derived from this source. This inosculation 

 does not, however, affect the validity of the forms in question, since they are the 

 result of individual variation in relative proportions of the different parts, the averages 

 showing decided differences. The females of many forms have not been examined 

 by me, while others are meagerly represented in the series examined, consequently it 

 has been deemed best not to attempt a "key" to them. 



