EXTERNAL FABTS OF BIRDS. — THE BILL. 
103 
sheath of either mandible may be pulled off whole, like the finger of a glove. It is, however, 
in many birds divided into parts, by various lines of slight connection, and then comes off in 
pieces ; as is the case with some water birds, particularly petrels, where the divisions are regu- 
lar, and the pieces have received distinctive names. Many auks {Alcidcc) have the covering 
of the bill in particular pieces, and it is an extraordinary fact that such parts are of a secondary 
sexual character (see p. 90), being assumed at the breeding season and afterwards moulted 
like feathers. Such condition of the sheath of the beak, or of special developments of tlie 
sheath, is called caducous or deciduous. The entire covering of both jaws together is called 
rhamphotheca (Gr. pafi(f)oi, hramphos, beak ; 6riKq, theJce, a sheath) ; of the upper alone, 
rhinotheca (Gr. pis, hris, the nose) ; of the under, gnatliotheca (Gr. yvaBos, gnatlios, jaw) ; but 
these terms are not much used, (c.) The covering is otherwise variously marked ; sometimes 
so strongly that similar features are impressed upon the bones themselves beneath. The most 
frequent marks are various ridges (Lat. pi. carince, keels) of all lengths and degrees of expres- 
sion, straight or curved, vertical, oblique, horizontal, lengthwise, or transverse; a bill so 
marked is said to be striate (Lat. stria, a streak) or carinate ; when numerous and irregular, 
they are called rugce (Lat. ruga, a wrinkle) and the bill is said to be corrugated or rugose^ 
When the elevations are in points or spots instead of lines, they are called puncta (Lat. punc- 
tum, a point) ; a bill so furnished is punctate, but the last word is oftener employed to designate 
the presence of little pits or depressions, as in the dried bill of a snipe towards the end. Larger 
softish, irregular knobs or elevations pass under the general name of tcarts or papillce, and a 
l)ill so marked is papillose ; when the processes are very large and soft, the bill is said to be 
carunculate (Lat. caro, flesh, diminutive carunculus, little bit of flesh). Various linear depres- 
sions, often but not always associated with carinae, are grooves or sulci (Lat. sulcus, a furrow) 
and the bill is then called sulcate. Sulci, like carinse, are of all shapes, sizes, and positions ; 
when very large and definite, they are sometimes called canaliculi, or channels. The various 
knobs, " horns," and large special features of the bill cannot be here particularized. Any of 
the foregoing features may occur on both mandibles, and they are exclusive of that special 
mark of the upper the nasal fossa in which the nostrils open, and which is considered below. 
We have still to notice the special parts of either mandible; and will begin with the 
simplest, the 
Under Mandible. — In the majority of birds it is a little shorter and a little narrower and 
not nearly so deep as the upper ; but sometimes quite as large, or even larger. The upper 
edge, double (i. e., there is an edge on both sides), is called the mandibular tomium, or in the 
plural, tomia (Gr. repveip, temnein, to cut ; fig. 26, j), as far as it is hard ; this is received 
against, and usually a little within, the corresponding edge of the upper mandible. The 
prongs already mentioned are the mandibular raini (pi. of Lat. ramus, a branch ; fig. 26, i) ; 
these meet at some point in front, either at a short angle (like >) or with a rounded joining 
(like P ). At their point of union there is a prominence, more or less marked (fig. 26, k) ; 
this is the gonys (corrupted from the Gr. yopv, gonu, a knee ; hence, any similar protuber- 
ance). That is to say, this point is gonys proper; but the term is extended to apply to the 
whole line of union of the rami, from gonys proper to the tip of the under mandible ; and in 
descriptions it means, then, the under outline of the hill for a corresponding distance (fig. 26, T). 
This important term must be understood; it is constantly used in describing birds. The 
gonys is to the under mandible what the keel is to a boat ; it is the opposite of the ridge or 
culmen of the upper mandible. It varies greatly in length. Ordinarily it forms, say, one- 
half to three-fourths of the under outline. Sometimes, as in conirostral birds, a sparrow for 
example, it represents nearly all this outline ; while in a few birds it makes the whole, and in 
some, as the puffin, is actually longer than the lower mandible proper, because it extends back- 
wards in a point. Other birds may have almost no gonys at all ; as a pelican, where the rami 
