4 NORTH AMERICAN BIRDS. 
b'. Tail more or less forked, the outer feather longest. 
ce. Tail much more than one-third as long as wing, usually (except in sub- 
genus Thalasseus) forked for more than one-fifth its total length, 
the outer feathers narrow and pointed at tips; webs of feet occu- 
pying more than half the interdigital space. 
ad. Depth of bill at base equal to one-third the length of the exposed 
culmen; gonys shorter than outer toe, without claw. 
Gelochelidon. (Page 38.) 
d. Depth of bill at base less than one-third the length of the exposed 
culmen; gonys longer than inner toe, without claw. 
Sterna. (Page 39.) 
‘e. Tail little more than one-third as long as wing, forked for less than one- 
fifth its total length, the outer feathers broad and rounded at tip; 
webs of feet occupying less than half the interdigital space. 
Hydrochelidon. (Page 46.) 
O, Tail praduated........ccccsscsssecsseccssscneecssescenecensceserens Anous. (Page 47.) 
Gznus GAVIA Borg. (Page 23, pl. VIIL, fig. 1.) 
Species. 
Adult; Entirely pure white, the shafts of the primaries pale yellowish ; bill yel- 
lowish; feet black. Young: Similar to adult, but quills, primary coverts, and tail- 
feathers each marked with a spot of dusky at tip, the lesser wing-coverts spotted 
with same. Length 15.00-19.50, wing about 13.25, culmen 1.40. Hab. Arctic 
Ocean, south, in winter, along Atlantic coast of North America to Newfoundland 
and New Brunswick..........:cccesscsssscessessessenees 39. G. alba (Gunn.). Ivory Gull. 
Genus RISSA Leacu. (Page 23, pl. VIIL,, fig. 2.) 
Species. 
Common CHaracters.—Adults with head, neck, rump, upper tail-coverts, tail, 
tips of secondaries, and entire lower parts pure white; mantle bluish gray, the 
quills varied with white and black; bill yellowish, feet blackish or bright red in 
life. Young similar to adults, but hind-neck crossed by a blackish collar or patch, 
and sometimes (in 2. tridactyla) a blackish patch on lesser wing-coverts and black 
band across tip of tail. Downy young white, tinged above with buffy and yellowish 
gray, but without spots or other distinct markings. Eygs 2-5, ovate, or short-ovate, 
olivaceous-white, grayish white, brownish white, or buffy, blotched and spotted 
with brown and lavender-gray. 
a’, Legs and feet black, or dusky. Summer adult: Pure white, the mantle deep 
pearl-gray ; five outer quills with terminal portion black, this decreasing from 
about 3.25 on the outer quill to .75 (more or less) on the fifth, the outer web 
of the first almost wholly black; the fifth, and sometimes the fourth, tipped 
with white. Winter adult: Similar, but hind part of head and neck washed 
