240 BIRDS OF ILLINOIS. 



Genus STERNA Linnaeus. 



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



Linn. 

 Ste.rnula Boie, Isis, 1822. 563. Type, Sterna minuta Linn. 

 Thalaseeus Boie, lsis, 18.'2. 563. Type. Sterna caspia Pall. 



Thalassea Kaup. Sk. En*w. Eur. Thierw. 1829, 97. Type. Sterna paradiscea Brunn. 

 Sylochelidun Bkehm. Vog Deutschl. 1830, 767. Type, Sterna caspia Linn. 

 A ctochelklon 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 Gelochelidon, 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 possibty 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. ( -iubgenus 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. 

 o. 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 weli-deflned stripe next the shaft, the inner edge 

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

 wholly black in spring, the forehea<l, lores, and centre of the crown white in the 

 breeding season. (Subgenus A cfochelidon Kaup.) 



2. S. maxima. Bill f-tout (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. sandvlcsnsis acuflaviius. 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. 

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



pale pearl-gray. (Subgenus Sterna Linn.) 

 11. 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. 



5. S. hirundo. Lower pats pale gray or grayish white; outer web of lateral tail- 



feather grayish or dusky, the inner web entirely white. 



