220 BIRD^ OF ILLINOIS. 



b l . Culmen decidedly more than two thirds as long as tarsus. 



c 1 . Tail even; size, color,. and all other characters extremely variable. 



3. Larus. 



c-. Tail emarginate or forked. 



4. Xema. Tail much less than half as long as wing, and forked for not more than 

 one eighth its length. 



5. Creagrus, Tail nearly half as long as wing, and forked for about one third its 

 length. (N ot represented in the Illinois fauna. ) 



6 2 . Culmen decidedly less than two thirds as long as tarsus. 



6. Rhodostethia. Tail graduated, the lateral feathers .75-1.25 shorter than the middle 



pair. (Not represented in the Illinois fauna.) 



SUBFAMILY STERNINJE. THE TERNS. 



A. Tail more or less forked, the outer feather longest. 



a. Tail much more than one third a^ long as wing, usually (except in subgenus 

 Tlialasseus) forked for more than one fifth its total length, the outer feathers 

 narrow and pointed at tips; webs of feet occupying more than half the interdigital 

 space. 



7. Gelochelidon. Depth of bill at base equal to one third the length of the exposed 

 culmen; gonys shorter than inner toe, without claw. 



8. Sterna. Depth of bill at base less than one third the length of the exposed cul- 

 men; gonys longer than inner toe, without claw. 



I). Tail little more than one third as long as wing, forked for less than one fifth 

 its total length, the outer feathers broad and rounded at tip; webs of feet oc- 

 cupying less than half the interdigital space. 



9. Hydrochelidon. 



B. Tail graduated. 



10. Anous. (Not represented in the Illinois fauna.) 



SUBFAMILY LARIN-3E. THE GULLS. 



GENUS RISSA LEACH. 



Rissa LEACH, Stephens's Gen. Zool. xiii, 1825, 180. Type, Larus rissa BBiiNN.=i. tri- 

 dactylus LINN. 



GEN. CHAK. Size medium; tail even, or slightly emarginate; hind toe rudimentary 

 or entirely absent, the nail usually obsolete; tars'us much shorter than the middle toe 

 without its claw, not rough or serrate behind. Adults with head, neck, rump, upper 

 tail- coverts, tail, tips of secondaries, and entire lower parts pure white; mantle bluish 

 gray, the quills varied with white and black; bill yellowish, feet blackish or bright red 

 in life. Yonng similar to adults, but hind-neck crossed by a blackish collar or patch, 

 and sometimes 'in R. tridactyla) a blackish patch on lesser wing-coverts and black baud 

 across tip of tail. Downy young white, tinged above with buffy and yellowish gi ay, 

 but without spots or other distinct markings. Eggs 25, ovate, or short-ovate, oliva- 

 ceous white, grayish white, brownish whit", or buffy, blotched and spotted with brown 

 and lavender- gray. 



