THE EXTERNAL STRUCIURE OF BIRDS. 147 
nasal fossa; if it is long and narrow it is called the nasal sulcus 
or groove. They are usually lateral in position, but are called 
culminal if they open on theculmen. When high, but not quite 
so high, they are superior, and they are inferior when nearer 
the maxillary tomium than the culmen, and they may be quite 
near the former—as in the Puffin. They are also usually basal 
or subbasal (7. e. situated near the base of the bill), but some- 
times they are median, as in the Goose. In the Apteryx they 
are terminal or at the end of the bill—a quite exceptional posi- 
tion. When the cere is feathered, they may be thus completely 
hidden, and they may be also hidden by the extension forwards 
of the frontal feathers of the head in pointed forward exten- 
sions (called antiw) on either side of the culmen—as in the 
Grouse. They may be also covered by setaceous feathers 
which project forwards over them, and therefore project in 
the contrary direction to that of the head-feathers generally, 
or are what is termed antrorse in direction. Very rarely, 
as in the Pelican, the nostrils do not open externally at 
all, or are impervious. It may be that they can both be 
seen through at the same time, or this may be impossible 
owing to the existence of a median septum between them. 
The terms perforate and imperforate are used to denote one or 
other of these conditions. The shape of the openings may 
vary from round to linear, and they are sometimes termed 
oblong, ovate, oval, or elliptic, as may be deemed the most fit. 
They are sometimes club-shaped or clavate. They may some- 
times have a raised rim round them much prolonged—as in the 
Petrels, and such are called tubular. Sometimes there is special 
development called a nasal scale, which may oyerarch the 
opening as in the Fowl, or may form a partial floor as in the 
Wryneck. The part of the bill between the nostrils is called 
the mesorhinum. 
The lower mandible is generally the smaller of the two, being 
slightly surpassed by the upper at the tip and also laterally. 
It is generally also less deep than the other. Its cutting-edge 
is called the mandibular tomiwm, which bites against, and 
generally a little within, the maxillary tomium of the upper 
mandible. Each half of the lower mandible is called a ramus, and 
the two ram unite at the tip and for a greater or less distance, 
thence backwards and downwards as the case may be. This 
line of junction forms part of the lower margin of the lower 
mandible, and corresponds with the culmen of the upper man- 
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