256 NORTH AMERICAN BIRDS. 
Ear-opening immense, extending almost the entire height of the skull, 
the two ears conspicuously asymmetrical......... Asio. (Page 257.) 
v%. Length of cere along top less than chord of culmen, the upper outline not 
arched. 
ce. Har-opening very large, with a distinct anterior operculum or “ flap,” 
the two ears conspicuously asymmetrical. (No ear-tufts.) 
@, Smaller (wing 12.00-15.00, tail less than 10.00); bill larger, more 
exposed ; at least the terminal scutelle of each toe exposed; 
eyes larger, with irides dark brown or nearly black; feet larger, 
and facial disk relatively smaller........ Syrnium. (Page 258.) 
d’. Larger (wing 16.00-18.00, tail 12.00 or more) ; bill relatively much 
smaller, and nearly hidden by feathers; toes entirely covered 
with feathers; eyes relatively much smaller, with irides yellow; 
feet relatively much smaller, and facial disk much broader. 
Scotiaptex. (Page 259.) 
ce. Har-opening small, without anterior operculum, or “ flap,” the two ears 
not distinctly asymmetrical. 
a. Har-tufts very conspicuous; tail reaching beyond tip of longest 
lower coverts ; two or three outer quills with inner webs emar- 
ginated ; toes covered with short but dense feathers, the claws 
wholly exposed ; bill exposed.................. Bubo. (Page 262.) 
@. Har-tufts rudimentary; tail not reaching beyond tips of longest 
lower coverts; four outer quills with inner webs emarginated ; 
toes covered with long hair-like feathers, partly or wholly con- 
cealing the claws; bill nearly concealed by the loral feathers. 
(Very large; wing 15.50 to 19.00.)........ Nyctea. (Page 264.) 
a’, Wing less than 10.00. 
b. Tarsus less than twice as long as middle toe, and much less than half as long 
as tail. 
c. Tail more than two-thirds as long as wing, graduated. (No ear-tufts; 
wing about 9.00.)...... cece ceeeeeeceeetseteneeeees Surnia. (Page 264.) 
c’. Tail less than two-thirds as long as wing, even, or slightly rounded. 
da‘, Kar-opening very large (nearly equal to height of skull), with ante- 
rior operculum or flap, the two ears conspicuously asymmetrical. 
(No ear-tufts; wing 5.25-7.40.).........4. Nyctala. (Page 260.) 
d’. Har-opening small, without anterior operculum, the two ears sym- 
metrical. 
e. Nostril large, oval, opening in anterior edge of cere; ear-tufts 
conspicuous; small (wing 5.40-7.80). 
Megascops. (Page 260.) 
é. Nostril small, circular, opening near the middle of the inflated 
cere; no ear-tufts; very small (wing 3.50-4.40). 
J*. Tarsus not longer than middle toe, densely feathered ; tail 
more than half as long as wing, rounded. 
Glaucidium. (Page 266.) 
