KEY TO GENERA, SPECIES, AND SUB-SPECIES 



i (42) Feathering of forehead projects into base of bill in the form of an 

 arch or promontory rounded at the end. 



2 (31) Feathering of forehead forms a regular arch; legs red, pink, 



or yellow (in young birds with more or less admixture of 

 grey), but not black. Feathering at sides of neck forms longi- 

 tudinal tufts, separated by deep furrows. Hind toe in walking 

 touches ground with claw. 



3 (6) Whole tail white, in young birds with faint grey tinge. Black 



streak stretches above edge of upper mandible and below edge 

 of lower from gape. Whole feathering, except black primaries, 

 white (adult bird) or light grey (young birds) ; legs and bill 

 red (adult) or grey (young birds). Genus Chen, Boie (1822). 



4 (5) Culmen 50-58 mm. (=1.96-2.28 in.); wing 370-435 mm. 



(= 14.50-17. 10 in.). C. hyperboreus, Pallas. 



5 (4) Culmen 63-70 mm. ( = 2.48-2.75 in.); wing 440-445 mm. 



(= 17.30-17.50 in.). C. nivalis, Forster. 



6 (3) Tail never wholly white ; no black streaks along edges of 



mandibles. White and pale grey are not prevailing tints in 

 plumage. 



7 (8) Tail, terminal half white, basal slate-colour. Length of nail on 



upper mandible contained in total length of culmen con- 

 siderably less than three times. Outer half or only outer 



margin of upper nail dark-coloured. Genus Philacte, Bannister (1870). 



Top and sides of head and back of neck white, 1 in sharp 

 contrast to dark lower part of head and foreneck (adult), or 

 head and neck slate-grey mottled with white (young birds). 

 Bill 35-42 mm. (1. 37-1. 65 in.); wing 350-400 mm. (= 13.70- 

 15.70 in.). P. canagica, Sevastianov. 



8 (7) Tail brown, but tail-feathers with white edgings and tips. 



Length of nail on upper mandible included at least three times 

 in total length of culmen. 



9 (30) Neck all round almost uniform grey-brown, only slightly darker 



behind. White not prevailing colour on head. 

 10(15) Whole bill light, from whitish, flesh-coloured, and pink, to 

 orange, with white nail (adult) or grey-fleshy tinge with grey 



1 Here the white is often coloured with a more or less intense yellow or rusty (sometimes amber-coloured) tinge, as for example 

 in the specimen figured. 



7 



