﻿58 



ON THE CLASSIFICATION OF BIRDS. 



of birds ; or 2. behind its basal margin, as in the 

 toucans ; or, 3. towards the very centre of the bill 

 — intermediate between the front of the head and the 

 tip of the under mandible, — as in the genus Musophaga 

 {fig. 33.). The first of these forms is common to most 

 birds, but there is yet great variation in the shape of 

 the aperture ; sometimes it is round, sometimes oval, 

 and sometimes linear (fig. 25. a). Round apertures 

 are most prevalent among the falcons, the owls, the 

 cuckoos, and the parrots, where the margins are 

 generally tumid or swelled. In the fern-owls (Ca- 

 primulgidce) , and in the cuckoo (fig. 24.), the aper- 

 ture is not only round, but slightly tubular, something 

 analogous to many of the bats. In the other fissirostral 

 birds, the opening is more oval, and is placed either at 



have also their own peculiar modifications. The grallatorial 

 birds show us this structure in its highest state of de- 

 velopment. On looking to the bill of the common 

 plover, we observe that there is a groove running in the 

 middle of the upper mandible of the bill for nearly two 



herons, this slit is without the groove ; while in the 

 humming birds (fig. 25. b) it is often so completely hid 



the extreme termination of 

 a membranaceous skin, which 

 fills up the depression of the 

 part where the nostrils are 

 situated, or a little on one 

 side: between these, and such 

 as are of a lengthened oval, 

 there are innumerable grades 

 of variation. Linear nostrils 



thirds its length; and that 

 the aperture of the nostrils 

 assumes the form of a mere 

 slit at the base (fig. 25. a). 

 A similar structure is seen 

 in the different snipes, cur- 

 lews, and sandpipers. In the 



