BIEL 33 
and the rostrum is the presence or absence of a complete vertical 
internasal septum. If the septum is complete, which seems to 
be the primary condition, the right and left nasal cavities are 
completely separated from each other, and birds having this 
structure are said to possess nares impervix. The septum either 
remains cartilaginous, or it ossifies to a variable extent. Con- 
sequently in macerated skeletons, where only the bony parts 
remain, this character cannot be determined. In comparatively 
few birds is the ossification complete, but this occurs in the 
Owls, in Podargus, in some Accipitres, Parrots, and others. When 
the septum is incomplete, the right and left nostrils communicate 
with each other, forming nares pervix, as in Phaethon, among the 
Steganopodes, in the Herons, Grebes, Divers, Gralle (except Rhino- 
chetus), Gavize, Limicole, Storks, Flamingos, Anseres, Cathartide 
(but not in the Vulturidz and Falconidz), and in many Passeres, 
especially in the Meliphagide. In some Steganopodes, for instance 
in the Cormorants, the nostrils are reduced to narrow slits, and this 
condition is carried to an extreme in the Gannets, the external 
nostrils being absolutely closed, and the greater portion of the nasal 
cavity obliterated or filled with cancellated bony tissue; how- 
ever, the olfactory apparatus is well developed, the inner nostrils or 
CHOANZ being very wide, and in open communication with the 
mouth, enabling the Gannet to smell its food when in the mouth. 
Various parts of the rostrum have received special names: 
culmen, the dorsal ridge of the upper bill; apex or tip; dertrum, 
in which it often terminates; gonys, or more correctly genys, the 
prominent ridge formed by the united halves of the under jaw, 
e.g. in Gulls ; tomia, the cutting edges of the bill. 
The form of the bill exhibits almost infinite variations in size, 
shape, and structure, of which only the most striking modifications 
can here be dealt with. Generally shape and size stand in obvious 
correlation with the mode of feeding, but sexual selection seems 
- also to play a great part, and leads to formations which it is often 
impossible to understand. 
The horny sheath of the bill sometimes consists of a number 
of pieces more or less separate. In the Ostriches and Tinamous 
there is a lateral pair and an unpaired piece for each jaw; in the 
Tubinares on the upper jaw at least one pair of lateral or maxillary 
pieces, an unpaired piece which covers the culmen and is continued 
into the prolonged nasal tubes, and an apical hook, strongly curved 
and pointed: each half of the under jaw is covered by one 
ventral, one dorsal, and one terminal piece, the latter partly fusing 
with that of the other side into a strong scoop. Indications 
of such a compound rhamphotheca are, however, found in other 
birds, especially in the Steganopodes, in some Herons, like 
Nycticorax and Scopus, and in Penguins; the culminar or dorsal 
VOL. I, 3 
