564 BULLETIN 50, UNITED STATES NATIONAL MUSEUM. 
ii. Much smaller (wing less than 350 mm.); bill relatively much 
smaller, much less strongly (sometimes hardly at all) decurved 
terminally, the gonydeal angle much less prominent, downy 
young spotted. 
j. Tarsus longer (usually much longer) than middle toe without 
claw, decidedly to much longer than exposed culmen; pri- 
maries partly black ‘‘patterned;” summer adults with white 
spots above and below eyes, or a white orbital ring; larger 
(wing 247-848 mm.)......-----.-.-- Chroicocephalus (p. 635), 
jj. Tarsus not longer than middle toe without claw, very slightly 
longer than exposed culmen; primaries without black 
“‘pattern;” summer adults without white about eye; very 
small (wing less than 225 mm.) .......Hydrocolzus (p. 649). 
gg. Adults with under parts wholly sooty brown or deep brownish gray 
(the head and neck sometimes white); young wholly sooty brown, 
the feathers of upper parts sometimes margined terminally or 
tipped with paler brown, whitish or buffy -..... Blasipus (p. 652). 
cc. Tail forked. 
d, Exposed culmen equal to or longer than tarsus, the latter not longer than 
middle toe with claw; bill deepest at base, distinctly decurved terminally; 
tail nearly half as long as wing, forked for about one-third its length; 
larger (wing 390-434, culmen 48.5-53); no black ring round neck. 
Creagrus (p. 658). 
dd. Exposed culmen decidedly shorter than tarsus, the latter decidedly longer 
than middle toe without claw; bill deepest at gonydeal angle, not de-. 
curved terminally; tail decidedly less than half as long as wing, forked 
for only about one-eighth its length; smaller (wing 260-285, culmen 
25-28.5); summer adults with a black ring round upperneck .Kema (p. 662). 
aa. Exposed culmen very little if any more than two-thirds as long as tarsus; tail 
cuneate or graduated, the middle pair of rectrices longest and much longer 
than:the: lateral: paltec.ccssew viene ikec ace eoanwasaaeeueen Rhodostethia (p. 668). 
Genus RISSA Stephens. 
Rissa StrpHens, Shaw’s Gen. Zool., xiii, 1826, 180. (Type, by monotypy, 
R. brunnichit Stephens=Larus tridactylus Linnzus.) 
Cheimonea Kavp, Nat. Syst. Eur. Thierw., 1829, 84,196. (Type, Larus tridactylus 
Linnzeus). 
[??] Polocandora ReicueNpacn, Nat. Syst. Vég., 1852 (1853), p. v. (Type not 
indicated.) 
Medium-sized Laride (wing, 293-330 mm.) with hallux absent or 
rudimentary; tarsus much shorter than middle toe without claw, 
not serrate behind; tail truncate or slightly emarginate, and young 
with a black nuchal patch (in one species with a black area on lesser 
wing-coverts and black band across tip of tail). 
Bill decidedly shorter than head (the exposed culmen decidedly 
shorter to very slightly longer than middle toe, without claw), 
rather stout, moderately compressed, the tip distinctly decurved, 
the culmen nearly straight basaliy but from above anterior end of 
nostril gradually curved to tip; gonys less than half as long as man- 
dibular rami, slightly but distinctly concave, its moderately promi- 
nent basal angle much anterior to anterior end of nostril; nostril 
with anterior end much posterior to middle of maxilla, Wing long 
