THE BXTBENAL STEtJCTUEB OP BIEDS. 143 



covering of the front part of the head may be replaced by a 

 horny plate or " shield," as in the Coot and Moor-hen. 



On the other hand, some or other of the head-feathers may 

 be exaggerated in size, forming crests, which may be single and 

 median as in the Cockatoo, or double and lateral, as in the 

 so-called "horns" of Eared Owls (structures which have 

 nothing whatever to do with the ears) and in the Crested 

 Grebe. 



Very rarely feathers of the mentum may be elongated, as in 

 the Bearded Vulture. More often those of the gular and 

 malar regions with the auriculars may form " ruffs." 



The Bill is perhaps the most important part of a Bird's ex- 

 ternal organization for the purposes of classification. It is also 

 a most important organ in the economy of the Bird's life, for it 

 not only serves for taking food, but, as already said, subserves 

 the purpose of a hand or fingers, and in some cases is an organ 

 of feeling, as in the Snipes and Woodcocks. The bill serves 

 for picking up, carrying, cutting, tearing, or crushing, accord- 

 ing to circumstances, and it is almost always more or less 

 conical, generally ending in a sharp point. 



Different definite technical terms are used by Ornithologists 

 to denote its form. A bill is said to be of medium length if it 

 is about as long as the head. If less than that length, it is 

 short, and it is long if it much exceeds it. A bill which is short 

 is said to be acute if pointed at the tip. If there is a hook- 

 like process at the tip, the bill is called hamulate or uncinate. 

 It is dentate when toothed as in a Falcon, and when there are 

 a number of small tooth-like processes along the margins of the 

 bUl, it is said to be serrate. If the bill, as in the Duck, bears a 

 nail-like process at the end of the maxilla, it is termed iingui- 

 eulate. When the bill is extremely long and slender as well as 

 pointed, it is sometimes compared with a needle and so called 

 aeicular, or, if less slender, to an awl, subulate. If only slightly 

 elongated it is acuminate ; and the term attenuate is supposed to 

 denote a condition intermediate between "acuminate " and " sub- 

 ulate." A bill which is flattened is said to be depressed, and if 

 widened at the end is called spatulate (as that of the Shoveller 

 Duck and, still more, that of the Spoonbill). A bill rather high 

 and narrow is called compressed. A bill of the most ordinary 

 shape, like that pf a Sparrow, is called conirostral. A beak which 

 is short with a wide gape, like that of the Swift, is termed _^ssi- 

 rostral. The quite opposite condition of bill («. g., that found in 



