126 ‘ GENERAL ORNITHOLOGY. 
happens in the larks (Alaudide) ; they are rare among land birds. common among waders. 
Those swimming birds with a very thin skinny podotheca are apt to show traces of the four- 
sidedness of the metatarsal bone. The tarsus in the vast majority of land birds is seen on 
close inspection to be somewhat ovate or drop-shaped on cross-section, — gently rounded in 
front, more compressed laterally, and sharp-ridged behind. This results from the laminiplan- 
tation described above, and is equally well exhibited by most passerine birds, whether they 
have booted or anteriorly scutellate tarsi. The line of union of anterior scutella with postero- 
lateral plates on the sides of the tarsus is generally in a straight vertical line, — either a mere 
Iie of flush union, or a ridge, or oftener a groove (well seen in the crows), which may or 
may not be filled in with a few small narrow plates. In the Clamatorial Passeres, represented 
by our flycatchers, the tarsus is enveloped in a scroll-like podotheca of irregularly arranged 
plates, the edges of the scroll meeting along the inner side of the tarsus. But the full consider- 
ation of special states of the tarsal envelope, however important and interesting, would be part. 
of a systematic treatise on ornithology, rather than of an outline sketch like this. 
The Number of Toes (individually, digiti; collectively, podiwm) is fowr: there are 
never more. There are two in the ostrich alone, in which both inner and hind toe are wanting. 
There are three in all the other struthious birds (Rheide, Casu- 
artide), excepting Apteryx, which has four. There are like- 
wise three, the hind toe being suppressed, in the tinamine 
genera Calodromas and Tinamotis (Dromeognathe) ; through- 
out the auk family (Aleide); in the petrel genus Pelecan- 
cides; apparently in the albatrosses (Diomedeine) ; usually in 
the gull genus Rissa; in the flamingo genus Phenicoparra ; 
throughout the bustard family (Otidide), and among various 
related forms, as Gdicnemus, HEsacus, Cursorius; in the 
plovers (Charadruda), excepting Squatarola; and in the 
Fig. 39.— Tridactyle foot of sand- bush-quails (Turnicide), excepting Pedionomus. In higher 
erling, Calidris arenaria; nat. size. yirds three toes are a rare anomaly, only known to occur in | 
three genera of woodpeckers (Picoides, Sasia, and Tiga), and in one galbuline genus (Jaca- 
maralcyon), by loss of the hind toe ; in two genera of kingfishers (Ceya and Alcyone), by sup- 
pression of the inner front toe; and in the passerine genus Cholornis, by defect of the outer 
front toe. North American three-toed birds are these only: the woodpeckers of the genus 
Picoides ; all auks (Alcide), and albatrosses (Diomedeine ; in these, however, there is a 
rudiment of the hind toe); all plovers (Charadriide, excepting one, Squatarola) ; the oyster- 
eatchers (Hematopus) ; the sanderling (Calidris, fig. 39); the stilt (Himantopus). Birds 
with two toes are said to be didactyle ; with three, tridactyle ; with four, tetradactyle. In the 
vast majority of cases, birds have three toes in front and one behind. Occasionally, either the 
hind toe, or the outermost front toe, is versatile, that is, susceptible of being turmed either 
way. Such is the condition of the outer front toe in ‘most owls (Striges), and in the fish-hawk 
(Pandion). We have no case of true versatility of the hind toe among North American birds; 
but several cases of its stationary somewhat lateral position, as in goatsuckers (Caprimulgide), 
some of the swifts (Cypselide), the loons (Colymbide), and all the totipalmate swimmers 
(Steganopodes). Nor have we any example of that rarest of all conditions (seen in some 
Cypselide, and the African Coliide) in which all four toes are turned forward. The arrange- 
ment of toes in pairs, two before and two behind, is quite common, being the characteristic 
of seansorial birds and some others, as all the parrots and woodpeckers, cuckoos, trogons, ete. 
Such arrangement is called zygodactyle or zygodactylous (Gr. (vydv, zugon, a yoke ; Sdxrudos, 
daktulos, a digit) ; and birds exhibiting it are said to be yoke-toed (fig. 45). In all yoke-toed 
birds, excepting the trogons, it is the outer anterior toe which is reversed ; in trogons, the 
