gs 
EXTERNAL PARTS OF BIRDS.— THE TAIL. 117 
twelve, of which the outer one on each side is spurious, very small, and hidden between the 
bases of the second and third feathers. Birds of prey (Raptores) have about twelve. In 
pigeons the rule is twelve or fourteen, as in all our genera; but sixteen are found in some and 
twenty in one case. In birds below these, the number increases directly; there are often or 
usually more than twelve in the grouse, and there may be sixteen, eighteen, or twenty, as 
among our own genera of Tetraonide. Wading birds, often having but twelve, furnish in- 
stances of as many as twenty. Those swimming birds with large well-formed tails, as the 
Longipennes, and some Anatide, have the fewest, as tweive, sometimes fourteen, rarely 
sixteen ; those with short soft tails have the most, as sixteen tu twenty-four. Among the 
penguins there are thirty-two or more. The Archeéopteryx appears to have had forty, —a pair 
to each free caudal vertebra; and this may be considered the prototypic relation between the 
bones and feathers of the tail. The 
Typical Shape of the Tail, as a whole, is the fan. The modifications of form, how- 
ever, which are greater and more varied than those of the wing, are susceptible of better 
definition, and many of them have received special names. Taking the simplest case, where 
the rectrices are all of the saine length, we have what is called the even, square, or truncate 
tail. The other forms depart from this mainly by shortening or lengthening of certain 
feathers. A tail nearly or quite even may have the two central feathers long-exserted, as seen 
in the jaegers (Stercorarius), and tropic-birds (Phaéthon). The most frequent departure from 
the even shape results from gradual shortening of successive rectrices from the middle to the 
outer ones. This is called, in general, gradation or graduation (Lat. gradus, a step); such 
shortening may be to any degree. More precisely, graduation means shortening of each 
successive feather to the same extent, —say, each half an inch shorter than the next; but 
such exactitude is not often expressed. When the feathers shorten by more and more, we 
have the true rounded tail, probably the commonest form among birds; thus, the gradation 
between the middle and next pair may be just appreciable, and then increase regularly to an inch 
between the next and the lateral feather. The opposite gradation, by less and less shortening, 
gives the wedge-shaped or cuneate (Lat. cuneus, a wedge) tail; it is well shown by the 
magpie (Pica) in which, as in many other birds, the middle feathers would be called long- 
exserted were the rest all as short as the outer one is. A cuneate tail, especially if the feathers 
be narrow and lanceolate, is also called acute, or pointed, as in the sprig-tailed duck (Dafila) 
or sharp-tailed grouse (Pediacetes). The generic opposite of the gradated is the forked tail ; 
in which the lateral feathers successively increase in length from the middle to the outermost 
pair. The least appreciable forking is called emargination, and a tail thus shaped is said to be 
emarginate ; when it is better marked, as, for instance, an inch of forking in a tail six inches 
long, the tail is truly forked or furcate (Lat. furea, a fork). But the degrees of fureation, like 
those of gradation, are so insensibly varied, that qualified expressions are usual; as, ‘ slightly 
forked,” “‘ deeply forked.” Deep fureation is usually accompanied by more or less narrowing 
or filamentous elongation of the lateral pair of rectrices, as in the barn swallows (Hirundo) 
and most of the sea-swallows (Sterna). An advisable term to express such an extreme furea- 
tion is forficate (Lat. forfex, scissors), when the depth of the fork is at least equal to the 
length of the shortest feathers ; it oceurs among our birds in those last named, in the species 
of the flycatcher genus Milvulus, and elsewhere. Double-forked and double-rounded tails 
are not uncommon; they result from combination of both opposite gradations, in this way: 
The middle feathers being of a certain length, the next two or three pairs progressively 
increasing in length, and the rest successively decreasing, the tail is evidently forked ceutrally, 
rounded externally, which is the double-rounded form, each half of the tail being rounded ; 
it is shown in the genera Myiadestes and Anous. Now if with middle feathers as before, 
the next pair or two decrease in length, and then the rest increase to the outermost, we have 
