BILL 35 



minuted particles of hard substance, or pulpy and soft food — and 

 also for filing or sharpening its mandible. 



In the Skimmer, Rhyncliop^, the bill forms two sharp vertical 

 blades, which somewhat gape asunder, with the further peculiarity 

 that the mandibular sheath and the suppoiting bone itself is con- 

 siderably larger than the upper portion. A vertically compressed 

 bill is also common in the Alcidoe, and is often vividly coloured 

 dui'ing the summer. In the Puffins the outermost bright layers 

 of the horny sheaths, and the horny excrescences at the gape of 

 the mouth and above the eyes are cast off periodically, these parts 

 being developed for the breeding season (Bureau, Bull. Soc. Zool. 

 France, 1877, p. 377/.) 



In many birds the covering of the bill, especially near the base 

 of the culmen and the forehead, is swollen, and forms various pro- 

 tubei-ances, horns, knobs, and other apparently ornamental excres- 

 cences. In the Coots and in Musophaga (Plantain -eater) 

 the coating of the culmen is produced backwards over the fore- 

 head, overlapping the latter as a conspicuous white or yellow 

 soft plate. Often the underlying bones, especially the nasals 

 and the adjoining premaxillary parts, are also swollen, and 

 form a light and extremely spongy meshwork of cancellated bony 

 tissue, a peculiarity Avhich attains its highest development in the 

 HORNBILLS and in the TouCANS. Similar swellings are the knobs 

 on the bill or on the forehead of the ScoTER and Mute Swan, of 

 Globicera among Pigeons, of certain Cracidse, and of Macrocephalon 

 (Megapode). In most of these cases the swellings are very light ; 

 rarely, as in the Helmet- Hornbill, the bones of the forehead 

 are greatly enlarged, and, although much cancellated, of great 

 weight and strength ; moreover, the horny epidermal covering of 

 the forehead is three quarters of an inch thick, and of the hardness 

 and weight of ivory. 



Another deviation is constantly found in the Crossbill's beak, 

 the sharply-pointed and hooked ends of the upper and lower jaws 

 crossing each other in an individually varying way, there being an 

 equal number of right and left -billed specimens. This crossing 

 begins to shew itself before the young birds are fledged, increases 

 with age, and ultimately leads to an asymmetrical development of 

 the masticatory muscles and of the bones of the occipito-quadrate 

 region. 



In Anarhynchus frontalis (Wrybill) the terminal half of the 

 bill is turned towards the right side, an abnormality which exists 

 in a marked degree even in the very young birds. The right 

 edges of the premaxilla and of the mandible are thin and strongly 

 turned inwards, so that the right and left sides are asymmetrical 

 in section. The left nostril and the groove which is continued 

 towards the terminal third of the bill remain in their original 



