564 BULLETIN 50, UNITED STATES NATIONAL MUSEUM. 



a. 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 orl)ital ring; larger 



(wing 247-348 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.) Hydrocolseus (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 decur\^ed 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 upper neck. Xema (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 pair Rhodostethia (p. 668). 



Genus RISSA Stephens. 



Rissa Stephens, Shaw's Gen. Zool., xiii, 1826, 180. (Type, by monotypy, 



R. bnmmchii Stephens=Lflrws tridactylus Linnaeus.) 

 Cheimonea Kaup, Nat. Syst. Eur. Thierw., 1829, 84, 196. (T>'p6, Lams tridactylus 



Linnaeus). 

 [??] Polocandora Reichenbach, Nat. Syst. Vog., 1852 (1853), p. v. (Type not 



indicated.) 



Medium-sized Laridse (wing, 293-330 mm.) with hallux absent or 

 rudimentary; tarsus much shorter than middle toe without claw, 

 not serrate beliind; 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 basally but from above anterior end ot 

 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 



