40 FORMS OF TAILS—THE FEET. 
some flycatchers, most of the terns, etc., etc. It would be advisable to have 
a term to express such extreme condition, which I shall call forficate, when 
the depth of the fork is equal to, or greater than, the length of the shortest 
(middle) pair of feathers ; it occurs among our birds in the genera Milvulus 
(no. 104), Sterna (291), and elsewhere. Double-forked or double-rounded 
tails are not uncommon; they result from combination of both gradation 
and forking, in this way : — Let the middle feathers remain constant, and 
the next two or three pairs progressively increase in length, then the rest 
successively decrease ; evidently, the tail is forked centrally, gradated exter- 
nally: this is the double rounded form; it is shown in the genera Myiadestes 
(no. 52) and Anous (294). Now with middle feathers as before, let the 
next pair or two decrease in length, and the rest progressively increase to 
the outermost: then we have the double-forked, a common shape among 
sandpipers. In the latter case, the forking rarely amounts to more than 
simple emargination, and generally is really little more than simple protru- 
sion of the middle pair of rectrices in an otherwise slightly forked tail ; and 
in neither case is the gradation either way often great. 
Various shapes of tails, which the student will readily name from the 
foregoing paragraph, are illustrated in figs. 17, 19, 29, 30, 32, 54, 57, 68, 
73, 76, 84, 98, 106, 117, 120, 121, 126, 133, 135, 137, 144, 145, 147-52, 
177, 206, 214. Ishould also allude to the folded tail of the barn-yard fowl 
(Gallus bankivi, var.) a very familiar but rare form. One of the most 
beautiful and wonderful of all the shapes of the tail is illustrated by the 
male of the famous lyre-bird (Menura superba), shown in the figure at the 
end of this Introduction. 
It should be remembered that to determine the shape, the tail should be 
viewed nearly closed ; for spreading will obviously make a square tail round, 
an emarginate one square, etc. I append a diagram of the principal forms. 
Fic. 7.— Diagram of shapes of tail. 
Fic. 7. adc, rounded; aec, gradate; aic, cuneate-gradate; alc, cuneate; abc, double- 
rounded; feg, square; fig, emarginate; fneog, double-emarginate ; kim, forked; kem, deeply 
forked; kb, forficate. 
Tue FEeEr. 
§ 71. In ALL Brrps, the posterior extremities are organized for progres- 
sion; for walking, hopping, or running on land, in all; but a few of the 
; 
