240 BIRDS OF ILLINOIS. 



Genus STERNA Linn^us. 



Slerna Linn. S. N. ed. 10, 1. 1758,137, ed. 12,i, 1766, 227. Type, by elimination. Sterna hirundo 



Linn. 

 Sternula BoiE. Isis, 1S22, 563. Type, Sterna minula LiNN. 

 2'halassevs Boie, Isis, 18.'2, 5ii3. Type. Sterna caspia Pall. 



Thalassea Kaup, Sic. En»w. Eur. Thierw. 182n, 07. Type. Sterna paradifcea Beunn. 

 Sulochelidun Crehm. Vog Deutsehl. 1830, 7li7. Type, Sterna caspia Linn. 

 yl c(oc7if(idnn Kaup, Sk. Ent. Eur. Thierw. 1829. 31. Type. Ste na canJmca GsiEi. 

 Haliplana Wagl. Isis, 1832. 1224. Type. Sterna fulignosa Gmel. 



Sp. Chae. Bill much more slender than In Gelochelidon, its depth through the base 

 less than one third the length of the exposed culraen, its upp"r outlines never strongly 

 curved and its lower outline always straight, or nearly so. Size exceedingly variable, the 

 form and colors less so; tail always decidedly forked, anl toes almost fully webbed, but the 

 webs concave, or "scalloped out," anteriorly. 



The above diafrnosis covers several fj^roups of species, usually 

 ranked as subgenera, some of which are possiblj' entitled to full 

 generic rank, since tiiey appear to be quite as distinct from typ- 

 ical Sterna as is Oelochelidon. 



The species occurring in eastern North America are the fol- 

 lowing: 



A . Wing more than 9. 00 inches. 



a. Size very large (wing IS, 00 inches or more); tail emarginate; occipital feathers soft and 



blended, not forming a crest: inner webs of primaries coneolored (dusky grayish); 

 adult, above, pale pearl-gray, beneath, white; hood wholly black in summer, wholly 

 streaked or speckled with white in winter. ( <ubf:enus Thalnsseus BoiE.) 



1. S. tsohegr va. Bill very robust (the depth through the base a little less than one 



third the length of the culmen). deep red. 



b. Size large or medium (wing 12.50-15 inches); tail deeply forked; occipital feathers 



pointed and somewhat lengthened, forming a short but distinct crest; inner webs 

 of primaries bicolored (dusky in a well-deflned stripe next the shalt, the inner edge 

 broadly and abruptly white); adult, pale pearl-gray above, white beneath; pilevim 

 wholly black in spring, the forehead, lores, and centre of the crown white in the 

 breeding season. (Subgenus Actor!ieUdoiiKii.vP.) 



2. S. maxima. Bill ^tout (but depth through the base much less than one third the 



length of th" culmen). deep orange; wing, 14.00-15. 0« inches. 



3. S. sandvioensis acuflavilua. Bill very slender, deep black, tipped with yellow. 



Wing, about 12.50 inches. 



c. Size small (wing less than 12.00 but more than 8.00 inches); tail of adult excessively 



forked, with lateral feathers very narrow towards tips; occipital feathers soft and 

 blended, not forming a crest. 



5. Upiier parts pearl gray, the whole top of head black in summer; tail white or 

 pale pearl-gray. (Sub.enus Sterna Linn.) 



H. Only one web of outer tail-feather entirely white. 



4. S. forsteri. Lower parts entirely white; outer web of lateral tail-feather entirely 



white, the inner web usually grayish or dusky near end. 

 6. S. hirundo. Lower pa ts pale eray or grayish white; outer web of lateral tall- 

 feather srayish or dusky, the Inner web entirely white. 



