584 BULLETIN 50, UNITED STATES NATIONAL MUSEUM. 
d. Larger, dept of bill at base usually more than 17.5 mm. 
Larus hyperboreus, young (p. 585). 
dd. Smaller, depth of bill at base usually much less than 17.5 mm. 
Larus leucopterus, young (p. 590).¢ 
bb. 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. 
f. Bill longer and heavier; middle toe without claw longer (culmen about 
63; middle toe about 65).......-.-.- Larus marinus, young (p. 601). 
Jf. Bill shorter and weaker; middle toe without claw shorter (culmen 
about 61; middle toe about 60). ...Lanus schistisagus, young (p. 606), 
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.> 
ii. Upper parts lighter, more of a hair brown. 
k. Bill heavier; middle toe without claw longer (depth of bill 
about 18; middle toe about 52)...........- Larus vegez.¢ 
Larus argentatus and subspecies, young (p. 612). 
kk. Bill slenderer; middle toe without claw shorter. 
Larus affinis, young (p. 608). 
hh, 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 
PASC. nis.s cic aicsisraisitrese’e Larus delawarensis, young (p. 624). 
i. Bill weaker, not more than 11 mm. in depth at base. 
Larus canus, young (p. 628).¢ 
LARUS HYPERBOREUS Gunnerus. 
“GLAUCOUS GULL. 
Adults in summer (sexes alike).—Head, neck, rump, upper tail- 
coverts, tail, and entire under parts, including axillars and under 
4D, kumlieni, L. nelsoni, and L. thayeri, immature plumages of which are not avail- 
able, probably come in here. 
b Extralimital. 
eh, argentatus (forms) and L. vege can not be distinguished in the immature plu- 
mages in a key. 
¢Immature plumages of LZ. canus and L. dalonontensis are very nearly alike and 
the depth of the bill at the base not always to be relied upon. 
