BIRDS OF NORTH AND MIDDLE AMERICA. 601 



Larus thayeri Brooks (W. S.), Bull. Mus. Comp. Zool., lix, no. 5, Sept., 1915, 

 373 (Buchanan 5ay, Ellesmere Land, Arctic America, coll. Mus. Comp. 

 Zool.). — Anderson, Summary Report Geological Survey of Canada for 1916, 

 1917, 377 (Bernard Harbor; Kater Point, Arctic Souml; Cape Kellett, 

 Banks Island). 



LaruR arrjentatus thayeri Dwight, Auk, xxxiv, 1917, 414, pi. 15 (localities; crit.). 



Larus argentaius Taverner, Summary Report Geological Survey of Canada for 

 1916. 1917, 356 (near Barkley Sound, Vancouver Island, B. C; crit.). 



LARUS MARINUS Linnaeus. 



GREAT BLACK-BACKED GULL. 



Adults in summer (sexes alike).' — Head, neck, rump, upper tail- 

 coverts, tail and entire under parts, including axillars and under 

 wing-coverts, immaculate pure white; back, scapulars, and wings 

 uniform dark sooty gray or blackish gray (between dark quaker 

 drab and deep slaty brown), the tertials and secondaries broadly 

 (the latter abruptly) tipped with white; outermost primary black, 

 tipped with white for about 63-75 mm., the second (from outside) 

 similar but more grayish basally and the white terminal area inter- 

 rupted by a broad black bar on one or both webs; third, fourth, and 

 fifth primaries becoming successively and more extensively, more 

 grayish basally, tipped with white and with an extensive black sub- 

 terminal area preceded by white, either in form of a spot or bar or 

 shading gradually into the dusky gray of proximal portion; remain- 

 ing (proximal) primaries dark sooty gray broadly tipped with white; 

 bill deep yeUow, paler at tip, the mandible red subterminally; rictus 

 orange-red or vermilion (in life); iris pale yellow, whitish or silvery; 

 bare orbital ring vermilion (in life) ; legs and feet pale flesh color. 



Adults in winter. — Similar to the summer plumage, but pileum and 

 hindneck streaked with dusky. 



Young. — Upper parts dark slaty brown or dusky, the feathers 

 broadly margined with pale duU buffy; remiges dusky or blackish, 

 narrowly tipped with whitish; tail dusk}^, narrowly tipped v/itli white 

 and crossed on distal portion by a narrow band of grayish or brownish 

 white; head, neck, and under parts dull whitish, the pileum, hind- 

 neck, and sides of head and neck streaked with dusky grayish brown, 

 the under parts (except throat) clouded or irregularly spotted with 

 grayish brown; bill blackish, sometimes paler basally; iris brown; 

 legs and feet dull whitish (in life) . 



Downy young. — General color dull grayish white, the upper parts 

 marbled or irregularly spotted with dull grayish; head marked with 

 irregular well-defined spots of black, as follows: Forehead with a 

 narrow mesial streak; crown with two spots, one behind the other, 

 with a minute spot on each side, opposite the space between the two 

 larger spots; occiput with four large spots, arranged in transverse 

 series, and below these three others, their position corresponding to 



