240 BIRDS OF ILLINOIS. 



GENUS STERNA LINN^US. 



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



LINN. 



Stc.rnula BOIE, Isis, 1822. 563. Type, Sterna minuta LINN. 

 '1 l,ala*neus BOIE, Isis, 18J2, 563. Type. Sterna caspia PALL. 



Thalassea KAUP. Sk. En'w. Eur. Thierw. 1829, 07. Type. Sterna paradlzcea BBUNN. 

 Sylochelidun BREHM. Vog Deutschl. 1830, 767. Type, Sterna caspia LINN. 

 A ctochelidon KAUP. Sk. Ent. Eur. Thierw. 1829, 31. Type, Ste. na cantiaca GMEL. 

 <Haliplana WAGL. Isis, 1832, 1224. Type. Sterna fulignosa GMEL. 



SP. CHAR. Bill much more slender than In GelocTielidon, its depth through the base 

 less than one third the length of the exposed culmen, its upper 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, and toes almost fully webbed, but the 

 webs concave, or "scalloped out," anteriorly. 



The above diagnosis covers several groups of species, usually 

 ranked as subgenera, some of which are possibly entitled to full 

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

 ical Sterna as is Gelochelidon. 



The species occurring in eastern North America are the fol- 

 lowing: 



A, Wing more than 9.00 inches. 



a. Size very large (wing 15.00 inches or more); tail emarginate; occipital feathers soft and 



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

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

 streaked or speckled with white in winter, (^ubgenus Thalasseus BOIE.) 



1. S. tschegr-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-defined stripe .next the shaft, the inner edge 

 broadly and abruptly white); adult, pale pparl-gray above, white beneath; pileum 

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

 breeding season. (Subgenus ActocJielidon KAUP.) 



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



length of the culmen). deep orange; wing, 14.00-15.00 inches. 



3. S. sandvicsnsis acuflaviius. Bill very slender, deep black, tipped with yellow. 



\Vintr, 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. 

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



pale pearl-gray. (Submenus 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 ur dusky near end. 



5. S. hirundo. Lower pa.ts pale gray or giayish white; outer web of lateral tail- 



feather grayish or dusky, the inner web entirely white. 



