BIRDS OF NORTH AND MIDDLE AMERICA. 459 



d. Feet with webs occupying riDre than halt th.^ length of interdigital spaces; 

 tail mnch more than one-third as long as vving, forked for more than one- 

 fifth its length, the outer pair of rectrices narrowed and more or less acu- 

 minate terminally. 

 e. Feathery of occiput elongated, forming a distinct erectile crest; bill rela- 

 tively larger or longer. 

 /. Occipital feathers broad and blsnded; tarsus decidedly longer than 

 middle toe with claw; tail forked for less than_ oae-fourth it-- length; 

 bill very stout, its greatest depth e([ual to about ouo-third the length 

 of exposed culmen; inner wel)s of primaries unicolorcd. 



Hydroprogne (p. 460). 



ff. Orcapital feathers narrow, acuminate (lanceolate); tarsus not longer 



thaa middle toe with claw; tail forked for about half its length; bill 



more slend3r, its greatest depth much less than one-third the length 



of exposed culmen; inner webs of primaries bicolored. 



Thalasseus (p. 406). 



ee. Feathers of occiput not elongated: Ijill relatively smaller. 



/. Bill relatively shorter and much stouter, its greatest depth equal to 



nearly one-third the length of exposed cidmen; gonys much shorter 



than mandibular rami; tail forked for only one-third its length; tarsus 



decidedly longer than middle toe with claw. ..Gelochelidon (p. 478). 



ff. Bill relatively longer and much more slender, its greatest depth less 



than one-fourth the length of exposed culmen; gonys nearly to quite 



as long as mandibular rami; tail forked for half its len<,th, or more; 



tarsus not longer (usually shorter) than middle toe with claw. 



g. Tail more than half as long as wing: size larger (wing 230 mm. or more). 



Sterna (p. 485). a 

 gg. Tail not more than half as long as wing; smaller (wing not more than 



180 mm.) Sternula (p. 520). 



dd. Feet with webs occupying less than half the length of the interdigital 

 spaces; tail little if any more than one-third as long as wing, forked for 

 less than one-fifth its length the lateral rectrices broad and rounded 



terminally Hydrochelidon (p. 526). 



cc. Frontal feathering advancing farthest anteriorly on or near base of culmen (if 

 forming an ob\ious lateral angle this very short and obtuse and placed much 

 above level of no.stril). 

 d. Frontal feathering slightly concave or indented at base of culmen; gonys 

 much shorter than mandibular rami; no plumes on sides of head; under 

 parts white. 

 e. Tail much more than half as long as wing, forked for more than half its 

 length, the lateral rectrices narrow and acuminate; inner webs of pri- 

 maries unicolored Seena (extralimital).^ 



ee. Tail much less than half as long as wing (but little more than one-third), 

 forked for less than one-third its length, the lateral rectrices broad 

 and only subacuminate at tips; inner webs of primaries bicolored. 



Phaetusa (p. 540). 



<» Including Onychoprion, which I am unable to separate generically. 



b Seena Blyth. Cat. Birds Mug. Asiat. Soc, 1849. 291 (type, by tautonymy, Sterna 

 seena Sykes). — Polamochelidon Heine, in Heine and Reichenow, Xomencl. Mus. 

 Hein. Orn.. 1890, 355 (to replace Seena Blyth, on grounds of purism. ITora/nos, 

 river; xe^t^cbi', a swallow; Richmond.) 



India to Malacca. (Monotypic.) 



