584 BULLETIN 50, UNITED STATES NATIONAL MUSEUM. 



d. Larger, depth of bill at base usually more than 17.5 mm. 



Larus hyperboreus, young (p. 585). 

 dd. Soaaller, depth of bill at base usually much less than 17.5 mm. 



Larus leucopterus, young (p. 590)." 

 bh. Tail with at least the terminal or subterminal third blackish or brownish, the 

 lateral feathers sometimes barred with white. 

 e. Culmen 60 mm. or more. 

 /. Bill longer and heavier; middle toe without claw longer (culmen about 



63; middle toe about 65) Larus marinus, young (p. 601). 



ff. Bill shorter and weaker; middle toe without claw sliorter (culmen 



about 61; middle toe about 60) Lanus schislisagus, young (p. 606j. 



ee. Culmen considerably less than 60 mm., not over 55. 

 g. Middle toe without claw over 45 mm. 

 h. Culmen 50 mm. or more. 

 i. Upper parts darker, more fuscous. 



j. Tarsi and feet (in dried skin) blackish brown. 



Larus occidentalis, young (p. 610). 

 jj. Tarsi and feet (in dried skin) light isabella color. 



Larus fuscus, young. f* 

 a. Upper parts lighter, more of a hah brown. 



h. Bill heavier; middle toe without claw longer (depth of bill 



about 18; middle toe about 52) Larus vegae.c 



Larus argentatus and subspecies, young (p. 612). 

 hh. Bill slenderer; middle toe witliout claw shorter. 



Larus affinis, young (p. 608). 

 /(//. Culmen considerably less than 50 mm. 



Larus californicus, young (p. 621). 

 gg. Middle toe without claw considerably less than 45 mm. 



h. Bill slenderer; dark tail band not so well defined and lighter 

 inclining to a more grayish brown (depth of bill at base 



about 10 mm.) Larus brachyrhynchus, young (p. 633). 



hh. Bill heavier; dark tail band more sharply defined and darker, 

 more inclined to blackish (depth of bill at base 11 mm. or 

 more). 

 i. Bill more robust, usually more than 11 mm. in depth at 



base Larus delawarensis, young (p. 624). 



a. Bill weaker, not more than 11 mm. in depth at base. 



Larus canus, young (p. 628). <^ 



LARUS HYPERBOREUS Gunnerus. 



GLAtrCOUS GTJLL. 



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

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



o L. kumlieni, L. nelsoni, and L. thayeri, immature plumages of which are not avail- 

 able, probably come in here. 



b Extralimital. 



c L. argentatus (forms) and L. vegx can not be distinguished in the immature plu- 

 mages in a key. 



d Immature plumages of L. canus and L. delawarensis are very nearly alike and 

 the depth of the bill at the base not always to be relied upon. 



