h. Bill with the mandibles much compressed, 

 the upper freely movable, the under con- 

 siderably exceeding the upper in length [p. 152. 

 (hypognathous) ; pterylosis as in Sterna . . RRYNCHOPIN^, 



c. Bill with the upper mandible long-er and bent 

 down over the tip of the inferior one (epi- 

 gnathous) ; tail usually square, seldom 

 forked, exceptionally cuueate LAEIN^, p. 161. 



Subfamily I. STERNIN^E. 



Key to the Genera. 



a. Tail from nearly square to deeply forked. 

 a'. Head without prolonged plumes at the 

 gape. 

 a". Tail-feathers rounded or slightly point- 

 ed ; tail short, less than half the length 

 of wing. 

 a'". Bill small, the culmen less than 

 twice the length of tarsus ; feet 

 feeble ; webs between toes consider- 

 ably indented 



6'". Bill very large and stout, culmen 

 more than twice the length of the 

 tarsus; feet moderately strong; webs 



only shghtly indented 



Outer tail-feathers longest, pointed. 

 c'". Tarsus very long, exceeding middle 

 toe and claw ; bill robust, obtuse ; 

 tail less than half the length of the 



b". 



[p. 5. 

 1. Hydrochelidon, 



2. Phaethusa, p. 23. 



d'". Tarsus short, less than middle toe 

 and claw. 

 a^. Tail very short, less than one third 

 of the wing; bill exceptionally 



stout and deep 



b*. Tail very long, more than three 

 fourths of the wing ; bill stout, 



the genys very short 



c*. Tail at least half — and generally 

 more than half — the length of the 

 wing ; bill compressed and slender ; 

 tarsus generally shorter than — and 

 never exceeding — the middle toe 



with the claw 



b'. Head with conspicuously prolonged plumes 

 at the gape ; bill stout, culmeu decurved ; 

 tail slightly forked, the two outer pairs of 



rectrices nearly equal in length 



b. Tail graduated, feathers pointed, the outer 



pair of rectrices shorter than the next pair. 



c'. Foot very long, the middle toe and claw 



exceeding the exposed culmen in length ; 



the second pair of rectrices from the 



outside the longest ; general colour grey. 



[p. 25. 

 8. Gelochelidon, 



[p. 32. 

 4. Hydroprogne, 



5. Seena, p. 37. 



6. Sterna, p. 41. 



7. N.ENIA, p. 132. 



[p. 133. 

 S. Procelsterna, 



