220 BIRD- OF ILLINOIS. 



h^. Culmen decidedly more than two thirds as long as tarsu?. 



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

 S. 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. (Not represented in the Illinois fauna.) 



52. Culmen decidedly less than two thirds as long as tarsus. 



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



pair. (Not represented in the Illinois fauna.) 



Subfamily STERNIN"^.— The Terns. 



A. Tail more or less forked. ihe outer feather longest. 



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

 Thalasseus) 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 Lnterdigital 

 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. 



b. Tall Uttle 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^.— The Gulls. 



Genus RISSA Leach. 



Bissa Leach, Stephens's Gen. Z06L xiii, 1825, 180. Type, Lams rissa Bkunn.=L. fri- 

 dactylus Linn. 



Gen. Chak. Size medium; tail even, or slightly emarginate; hind toe rudimentary 

 or entirely absent, the nail usually obsolete; tarsus much shorter than the middle toe 

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

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

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

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

 and sometimes in li. tridactyla) a blackish patch on lesser wing-coverts and black band 

 a 'ross tip of tail. Downy young white, tinged above with buffy and yellowish g^ay, 

 but without spots or other distin .'t markings. Eggs i—h. ovate, or short-ovate, olive- 

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

 and lavond<;r-Kray. 



