132 GENERAL OBNITHOLOGY. 



happens in the larks (Alaudida) ; 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 fi-om 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 genctrally in a straight vertical line, — either a mere 

 line of Hush 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, podiuiii) is four : 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 (Rheidce, Casu- 

 ariida), excepting Ajiteri/x, which has four. There are like- 

 wise three, the hind toe being suppressed, in the tinamine 

 genera Calodromas and Tinamotis {Dromceognathce) ; through- 

 out the auk family {Alcklce) ; in the petrel genus Pelecan- 

 o'ides; apparently iu the albatrosses (Diomedeina;) ; usually in 

 the gull genus Eissa; in the flamingo genus Phoenicoparra ; 

 throughout the bustard family (Otididee), and among various 

 related forms, as (Edicnemus, Esacus, Cursorius ; in the 

 plovers (Charadriidcs), excepting Squatarola ; and in the 

 Fig. 39. — Tridactyle foot of sand- bush-quails (Tumicidce), excejsting Pedionomus. In higher 

 erling, Calidris arenaria ; nat. size. ^^.^.^^^ ^j^^^^ ^^^^ ^^^ ^ ^^^^ anomaly, only known to occur in 



three genera of w^oodpeckers (Picmdes, Sasia, and Tiga), and in one galbuline genus (Jaca- 

 maralcyon), by loss of the hind toe ; in two genera of kingfishers {Ceyx 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 

 Pico'ides ; all auks (Alcidce), and albatrosses (Dioviedeince ; in these, however, there is a 

 rudiment of the liind toe) ; all plovers (Charadriidce, excepting one, Squatarola) ; the oyster- 

 catchers (Hcematopus) ; the sanderling (Calidris, fig. 39) ; the stilt (Himantopus). Bu'ds 

 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 turned 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 {Caprimidgido'), 

 some of the swifts {CypselidcB) , the loons (Colymbidce) , and all the totipalmate swimmers 

 (Steganopodes). Nor have we any example of that rarest of all conditions (seen in some 

 Cypselidce, and the African Coliidaf) 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 scansorial birds and some others, as all the parrots and wood})eckers, cuckoos, trogons, etc. 

 Such arrangement is called zygodactyle or zygodactylous (Gr. (vyov, zugon, a yoke ; haKTvKoi, 

 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 



