THE EXTERNAL STRUCTURE OF BIRDS. 161 
Length of Digits—Of the three external digits the median 
one is almost always the longest; the toes on either side of this 
are generally of equal length, but if unequal, then, save in rare 
instances, the outer of the two is the longer. 
The connection between the digits may be confined to the imme- 
diate proximity of their origin, when they are separate and dis- 
tinct, as in the Thrush. The third and fourth toes may cohere 
for almost their whole length, being invested by skin as if they 
were but one toe. This condition is termed syndactyle or aniso- 
dactyle or syngenesious. Examples of this structure are to be 
found in Kingfishers and Hornbills. The two outer digits 
may be exceptionally connected in some Fowls and Pigeons *. 
The digits are often much connected together by folds of 
skin, or are palmate. Such Birds are said to be web-footed, as 
in the Duck. The webs ordinarily connect together only 
the 2nd, 3rd, and 4th toes, but they may, in addition, con- 
nect the first toe with the second—as in the Pelican and 
Cormorant. Such a foot is called totipalmate. If the webs 
are short, only reaching about halfway down the toes or less, 
the foot is semipalmate. The webs may have their margins 
sharply concave or cut in; such are said to be incised. The 
incision may extend to the roots of the toes, so that each digit 
is fringed with membrane, which forms festoons on either side 
of it. A foot of this kind is said to be lobate, and we find such 
in the Coot and Grebe. 
The skin of the pelvic limb is always more or less invested 
with feathers, but also almost always partly bare. The skin of 
the bare portion is peculiarly conditioned and needs separate 
description ; but the plumage had better first be noticed. 
The feathers of the pelvic limb are very different from those 
of the pectoral one. Only by very rare exception, and as a sort 
of monstrosity, are long pennaceous feathers developed, com- 
parable with the -remiges of the wing. The thigh is always 
feathered as part of the body-plumage. The feathers of the 
crus are almost always inconspicuous, but they may be con- 
spicuous from their length, as in the Hawks and some Cuckoos. 
The crus is always feathered at its upper part, and may be 
feathered its whole length, as in the Diver. 
The tarsus is generally naked, but it may be closely invested 
* Tn some of those which have abnormally developed feathers, technically 
ealled ‘‘ boots.” See below, p. 162. 
M 
