404 Snodgrass, Geospiza. Cocortiis, and CcrtJiidia. L()ct^ 



group, and it is between the former and Ccrthidia that Coconiis is 

 really intermediate. The theory above suggested involves the 

 assumption that the four groups of Geospiza became differentiated 

 in color before the great variation in the bill took place, and con- 

 sequently, that Coconiis branched off from one of the melanistic 

 groups comparatively late in Geospizan history. Such an assump- 

 tion is entirely in harmony with the facts discussed by Mr. 

 Edmund Heller and the writer in the paper referred to above in 

 the footnote. The fact that large groups of Geospiza are defi- 

 nitely characterized by color, while there is an infinite amount of 

 specific and subspecific variation in the bill, would indicate that 

 the color differences were evolved and stereotyped long before 

 the bill variation began. 



What immature stages of Coconiis are known indicate that the 

 adult males reach the black phase through paler phases similar to 

 those of Geospiza. Certhidia, both in the adult and immature 

 stages, is of a plain, pale and comparatively uniform coloration. 



I. The Skull. 



In order to show more strikingly the wide departure that the 

 Geospiza skull makes from the ordinary Passerine type, a descrip- 

 tion of one of the most specialized forms will be given first. The 

 descriptions of other species follow in the order of decreasing 

 modification. 



Geospiza strenua Gould (Plate XVII, Figs, i and 2). — Ridges 

 of sides and posterior part of skull strikingly prominent ; temporal 

 crests parallel ; interorbital area on top of skull parallel-sided and 

 almost as wide as the inter-temporal area. In general the cranium, 

 viewed from above, has a curious resemblance to a rodent's skull. 



Top of head very smooth. Highest point between tips of squa- 

 mosal processes ; profile descending from here to lambdoidal crest 

 in a regular curve, to base of culmen less steeply and in more 

 nearly a straight line. Space between temporal crests transversely 

 flat, i. e., the crests are coincident with the dorsal profile. Inter- 

 orbital area much wider than long, depressed mesially and decli- 

 vent toward each lateral margin, the anterior margin nearly 

 straight. Postfrontal process large, trihedral and unciform. Tem- 



