48 NUMBER AND POSITION OF TOES. 
compressed: the form is seen in its highest development in the loon, where 
the tarsus is almost like a knife-blade. Cylindrical tarsi occur chiefly when 
there are scutella before and behind; it occurs in our shore lark (Hremo- 
phila, gen. 26), but is a rare modification among land birds, though very 
common among waders. The tarsus of the vast majority of land birds is 
seen, on close inspection, to be sharp-ridged behind, and gently rounded in 
front. This generally results from the presence, in front, of a series of 
scutella, associated, on the sides and hinder edge of the tarsus, with fusion, 
or with a few large plates variously arranged. The meeting of these two 
kinds of envelope on the sides of the tarsus is generally in a more or less 
complete straight up and down line; either a mere flush trace of union, or 
a ridge, oftener a groove (well seen in the crows) that may or may not be 
filled in with a few small linear plates. But further consideration of special 
states of the tarsal envelope, however important and interesting, would be 
part of a systematic treatise, rather than of an outline sketch like this. 
§ 84. Tue Tors (individually, digiti; collectively, podium). Their nor- 
mal number is Four: there are never more. The ostrich alone has only two. 
There are three in all the auks (fam. Alcide) and albatrosses (subfam. Dio- 
medeine) ; in all struthious birds, except the ostrich and Apteryx; and ina 
large number of waders (Galle). Three toes only occur as an anomaly 
among Insessores, as in the cases of the exotic genus Ceyx of kingfishers, 
and the genus Picoides of woodpeckers. North American three-toed birds 
are only these : —the woodpeckers just named ; auks and albatrosses ; plovers 
(except one, Sqguatarola, 189); the oystercatchers (Hamatopus, 194) ; the 
sanderling (Calidris, 211); the stilt (Himantopus, 197). In the vast 
majority of cases, there are three toes in front, and one behind; occasionally, 
either the hind one, or the outermost front one, is versatile, that is, capable of 
being turned either way ; the outermost one is mostly so in the owls, the fish 
hawk (gen. 153), and a few other birds. We have no case of true versatility 
of the hind toe among North American birds, but several cases of its lateral 
stationary position (goatsuckers, some Western swifts, loons, and all the tot- 
ipalmate swimmers); nor have we any example of that rarest condition 
(seen in the European swifts, Cypselus, and in the Coliide) where all four 
toes are turned forward. This only occurs in the order Sérisores. The ar- 
rangement of toes éwo in front, and ¢wo behind, or in pairs, characterizes the 
whole order Scansores, or climbers; such birds are said to be zygodactylous 
(yoke-toed ; see fig. 128). Our examples are the parrot, woodpeckers and 
cuckoos, to which some add the trogons; in all these, except the last named, 
it is the outer anterior toe that is reversed. In nearly every three-toed bird, 
all three are anterior; our single exception is the genus Picoides (132), 
where the hind toe is wanting, the outer anterior reversed to take its place, 
and only two left in front. No bird has more toes behind than in front. 
All birds’ toes are 
§ 85. NuMBprreED, in a certain definite order, as follows (see figs. 8, 9) :— 
hind toe (1t) = first toe; inner anterior toe (2t) = second toe; middle an- 
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