ORIGIN AND CHARACTERISTICS 39 



more popular treatment, and is so closely con- 

 nected with the habits and characteristics of 

 birds that some amount of detailed description 

 becomes absolutely necessary in a work of the 

 present scope. A Beak (rostrum) consists of 

 an upper jaw or Maxilla, and a lower jaw or 

 Mandible. The bill is divided into several parts 

 by anatomists, chief of which are the culmen or 

 dorsal ridge of the upper bill, the apex or tip, 

 the dertrum or swollen hook in which it often 

 terminates, the genys or ridge or angle of the 

 lower mandible, and the tomia or cutting edges 

 of the bill. The horny sheath or rhamphotheca 

 in which the bones of the bill are enclosed in 

 some cases consists of a number of pieces, but 

 in the majority of cases forms one coherent 

 sheath. In birds of the Duck tribe most of the 

 covering sheath is soft, only the tip or nail being 

 hard and horny ; whilst in Raptorial birds and 

 Parrots the distal end of the bill is hard and the 

 basal portion is soft and forms a cere, in which 

 the nostrils are generally situated. The nasal 

 portion of the bill in many birds is soft and 

 furnished with an operculum, as in the Pigeons ; 

 whilst in the Petrels this operculum is still more 

 developed and forms a tube. In some birds the 

 horny sheath is curiously modified and swollen 



