BIRDS OF NORTH AND MIDDLE AMERICA. 459 
d. Feet with webs occupying more than half the length of interdigital spaces; 
tail much more than one-third as long as wing, forked for more than one- 
fifth its length, the outer pair of rectrices narrowed and more or less acu- 
minate terminally. 
e. Feathers of occiput elongated, forming a distinct erectile crest; bill rela- 
tively larger or longer. 
f. Occipital feathers broad and blended; tarsus decidedly longer than 
middle toe with claw; tail forked for less than oue-fourth its length; 
bill very stout, its greatest depth equal to about one-third the length 
of exposed culmen; inner webs of primaries unicolored. 
Hydroprogne (p. 460). 
ff. Occipital feathers narrow, acuminate (lanceolate); tarsus not longer 
thaa middle toe with claw; tail forked for about half its length; bill 
more slend2r, its greatest depth much less than one-third the length 
of exposed culmen; inner webs of primaries bicolored. 
Thalasseus (p. 466). 
ee. Feathers of occiput not elongated; bill relatively smaller. 
j. Bill relatively shorter and much stouter, its greatest depth equal to 
nearly one-third the length of exposed culmen; 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). 
Jf. 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 length, 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).¢ 
gg. Tail not more than half as long as wing; smaller (wing not more than 
T80 Mt) ssex cance sedss ve seated sceeeeaccenas 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 wiag, forked for 
less than one-fifth its length the lateral rectrices broad and rounded 
PSrMNAAY oa as conse owns emcwee ne aeebe eine wa Hydrochelidon (p. 526). 
cc. Frontal feathering advancing farthest anteriorly on or near base of culmen (if 
forming an obvious lateral angle this very short and obtuse and placed much 
above level of nostril). 
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. 
Phaétusa (p. 540). 
@ Yacluding Onychoprion, which I am unable to separate generically. 
6 Seena Blyth, Cat. Birds Mus. Asiat. Soc., 1849, 291 (type, by, tautonymy, Sterna 
seena Sykes).—Potamochelidon Heine, in Heine and Reichenow, Nomencl. Mus. 
Hein. Orn., 1890, 355 (to replace Seena Blyth, on grounds of purism. Morass, 
river; xeAdav, a swallow; Richmond.) 
India to Malacca. (Monotypic.) 
