BILL 35 
minuted particles of hard substance, or pulpy and soft food—and 
also for filing or sharpening its mandible. 
In the Skimmer, Lhynchops, the bill forms two sharp vertical 
blades, which somewhat gape asunder, with the further peculiarity 
that the mandibular sheath and the supporting bone itself is con- 
siderably larger than the upper portion. A vertically compressed 
bill is also common in the Alcidz, and is often vividly coloured 
during 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 ff) 
In many birds the covering of the bill, especially near the base 
of the culmen and the forehead, is swollen, and forms various pro- 
tuberances, horns, knobs, and other apparently ornamental excres- 
cences. In the Coors 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 which attains its highest development in the 
HornBIL1s and in the Toucans. Similar swellings are the knobs 
on the bill or on the forehead of the ScoTER and Mute Sway, of 
Globicera among Pigeons, of certain Cracide, 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, an, 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 (WRyYBILL) 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 
