HIRUlSTDINIDiE. 
admitted that if followed strictly still further subdivision would be necessary 
and peculiarly enough this has taken place partly on the lines suggested 
by them. 
As diagnostic of the family they gave the following three features : one 
moult, nine primaries, nestling with spinal feather-tract bifurcated ; but 
again,; they admitted that the first character might not apply to extra- 
Palaearctic and Nearctic forms. 
The very long, narrow wings, long forked tail and small bill with large 
gape which characterise the typical species superficially vary only in the 
depth of the tail-forking, the other features making any species recognisable 
at sight. 
vol. Yin. 
33 
