BILL 33 



and the rostrum is the presence or absence of a complete vertical 

 iiiternasal septum. If the septum is complete, which seems to 

 be the primary condition, the right and left nasal ca^dties are 

 completely separated from each other, and birds having this 

 structui'e are said to possess nares impervige. 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 occm's 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 pervise, as in Phaethon, among the 

 Steganopodes, in the Herons, Grebes, Divers, Grallse (except Ehino- 

 chetus), Gavipe, Limicolse, Storks, Flamingos, Anseres, Cathartidse 

 (but not in the Vulturidse and Falconidse), and in many Passeres, 

 especially in the Meliphagidse. In some Steganopodes, for instance 

 in the Cormorants, the nostrils are reduced to naiTow 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 

 ChoaN-E being very ^vide, 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 : 

 ciilmen, the dorsal ridge of the upper bill ; apex or tip ; dertrmn, 

 in which it often terminates ; goni/s, 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 vai-iations 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 mth 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 Avhich 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 j^artly fusing 

 Avith 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 



