of Colymbus adamsi m Norway. 277 



less, its total length being often under 800 mm., and 

 seldom exceeding 850 mm. The size in both species varies, 

 however, to a considerable extent, and small specimens of 

 C. adamsi may scarcely exceed C. ylacialis in size. 



(2) The Shape and Length of the Bill. 

 Whilst in C. adamsi the culmen is straight, and the com- 

 missure even slightly bent upwards, the culmen in C. glacialis 

 is more or less distinctly curved, and the commissure is 

 seldom perfectly straight. The margins of the bill of 

 C. adamsi are but slightly curved inwards ; in old specimens 

 that of the lower mandible becomes much worn and jagged. 

 On the whole the bill is clearly larger and stronger. In a 

 large specimen of C. glacialis the culmen has a length of 

 86 mm., whilst its average length in C. adamsi is 92 mm., 

 and occasionally reaches 98 mm. In mature specimens of 

 the last-named species the culmen is longer than the tarsus ; 

 in C. glacialis it is shorter. The angle at the symphysis of 

 the lower mandible is, as a rule, more acute in C. adamsi, 

 and the line from the gonys to the tip of the bill is qiiite 

 straight or even concave. But this character varies, and in 

 some specimens of C. adamsi the gonys is but slightly more 

 prominent than in C. glacialis. 



(3) The Colour of the Bill. 



The colour of the bill in C. adamsi, even in young birds of 

 the year, is whitish, a little dusky at the base. As soon as the 

 bill is fully developed in the one-year-old birds, it has already 

 nearly assumed the colour of that of the old birds, which is 

 whitish yellow, but always with a tinge of leaden grey or 

 brown horn-colour near the base. In fresh individuals, as a 

 rule, there is also a horn-coloured portion surrounding the 

 nostrils. In the full nuptial plumage the bill becomes still 

 more bleached, and there only remains a narrow leaden-grey 

 margin along the base of the lower mandible. 



In summer-plumage the colour of the bill of C. glacialis 

 is black or blackish; in the young birds during the first 

 winter horn-colour. Examples of both species may there- 



SKR. VI. VOL. VI. u 



