THE EXTBENAIi STEtTCTtJEE OF BIEDS. I45 



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 metagnaihous. 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. Barest of all is the form denominated Jiypo- 

 gnathous, in which the lower mandible is longer than the upper, 

 in the Skimmer (Bhynchops) 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 bUl, 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 homy bill, and these two junctions re([uire 

 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 maxilla 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. 



J This distinction was proposed by Elliott Coues in the second edition of 

 his ' Key to North American Birds,' p. 105. 



Ii 



