THE EXTERNAL STRUCTURE OF BIRDS. 145 
where its mobility is very marked. These differences depend on 
conditions of the skeleton, to be noticed later on. 
The two mandibles are almost always of about equal length, 
meeting together at the point and not overlapping. In the 
great majority of birds the lower mandible fits into the upper 
one. This normal condition is termed paragnathous. When 
the mandibles cross each other towards their apices, as in the 
Crossbill*, they are termed metagnathous. This is an extremely 
rare condition, but it is by no means unusual to find the upper 
mandible longer than the lower one, and curving over the tip of 
the latter, as in Parrots and Hawks. This condition is termed 
epignathous. Rarest of all is the form denominated hypo- 
gnathous, in which the lower mandible is longer than the upper, 
in the Skimmer (Rhynchops) f. 
The two mandibles join each other so as to form by their 
junction a line, which may or may not be straight, and which 
extends from the tip of the bill back to the point where the two 
jaws laterally unite, which is the angle of the mouth. This line 
is formed in part (anteriorly) by the junction of the upper and 
lower portions of the horny bill, and in part (posteriorly) by 
the junction of the two jaws when their opening extends back- 
wards beyond the hinder end of one or both portions (upper 
and lower) of the horny bill, and these two junctions require 
distinguishing by two different names. The former one is the 
tomia ¢, the latter one is the rictus, while the whole margin of 
the tomia and rictus, taken together, is the gape or commissural 
line, or line of commissure. The angle of the-mouth may be 
further distinguished as the point of commissure. The line 
of commissure, or gape, may be straight, curved, or angulate. 
It will be “straight ” when the tomia and the rictus together 
form one straight line. It will be “ curved,” or sinuate, when 
they together form one curved line. Finally, it will be 
* angulate” when both the tomia and the rictus are nearly 
straight but do not lie in one line, and, therefore, form an angle 
at their point of junction, . 
The mawilla or upper mandible has two definite lines most 
evident to the observer when the bill is viewed in profile. One 
of these lines coincides with the uppermost margin of the bill 
* See p. 104. 
t See above, p. 30. 
{ This distinction was proposed by Elliott Coues in the second edition of 
his ‘ Key to North American Birds,’ p. 105. 
L 
