EXTERNAL PAMTS OF BIRDS. — THE BILL. 103 



sheath of either maiuliblo may be pulled ott' whole, like the finger of a glove. It is, however, 

 iu many bu'ds divided into parts, by various lines of slight connection, and then comes off in 

 i)ieces ; 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 cliaracter (see p. 90), being assumed at the breeding season and afterwards moulted 

 like feathers. Such ct>nditiou of the sheath of the beak, or of special developments of the 

 slieath, is called caducous or deciduous. The entire covering of both jaws together is called 

 rhamphutheca {Gr. pafKpos, hmmphos, beak ; diiKrj, theJce, a sheath) , of the upper alone, 

 rhinotheca (Gr. pis, hris, the nose) ; of the under, gnathotheca (Gr. yvadoi, gnathos, jaw) ; but 

 these terms are not much used, (c.) The covering is othenvise variously marked ; sometimes 

 so strongly that similar features are impressed upon the bones themselves beneath. The most 

 frequent marks are various ridges (Lat. pi. carinas, 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 rugcs (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. pune- 

 tum, a point) ; a bill so furnished is punctate, but the last word is oftener employed to designate 

 the presence of little jiits 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 uarts or papillts, and a 

 bill so marked is papillose ; when the processes are very large and soft, the bill is said to be 

 caruneulate (Lat. caro, flesh, diminutive carunculus, little bit of flesh). Various linear depres- 

 sions, often but not always associated with carina, are grooves or sulci (Lat. sidcus, a furrow) 

 and the bill is then called sulcate. Sulci, like caringe, 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 wiU begin with the 

 simplest, the 



Under Mandible. — Iu 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 tomiiim, or in the 

 plural, tomia (Gr. refivetu, 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 rami (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 D ). At their point of union there is a prominence, more or less marked (fig. 26, k) ; 

 this is the Goxvs (corrupted from the Gr. -ydio;, 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, fr(nu gonys proper to the tip of the under mandible ; and in 

 descriptions it means, then, the under outline of the hill for a coiTCspouding distance (fig. 26, T). 

 This important tenn must be understood ; it is constantly used iu 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 fonns, say, one- 

 half to tliree-fomths of the under outline. Sometimes, as in conirostral birds, a sparrow foi 

 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 ahnost no gonys at all ; as a pelican, where the rami 



