SEC. Ill EXTERIOR PARTS OF BIRDS 187 



tarsal envelope, however important and interesting, would be part 



of a systematic treatise on ornitliology, rather than of an outline 



sketch like this. 



The Normal Number of Toes (individually, digiti; collectively, 



podium) 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 



{Bheidm, Casuariidce), excepting ^p^eryx, 



which has four. There are likewise 



three, the hind toe being suppressed, in 



the tinamine genera Calodromas and 



Tinamoiis (Dromceognathce) ; throughout 



the auk family (Alcidce) ; in the petrel 



genus Pelecanoides ; apparently in the 



albatrosses (Diomedeince ; in these, 



however, there is a rudiment of the 



hind toe) ; usually in the gull genus 



iJfea; in the flamingo genus Fhcenico- ^yii'g^cSidHs'attS^^^nLtzt'^'^' 



parra; throughout the bustard family 



(Ottdidm), and among various related forms, as CEdicnemus, 



Hmmatopus, Himantcpus, Esacus, Oursorius ; in the plovers (Chara- 



driidoB), excepting Sguatarola ; in the sandpiper genus Calidris ; and 



in the bush-quails (Turnkidce), excepting Pedionomus. In higher 



birds three toes are a rare anomaly, only known to occur in three 

 genera of woodpeckers {Pkoides, Sasia, and Tiga), and in one galbu- 

 Hne genus (Jacamaralcyon), by loss of the hind toe ; in two genera 

 of kingfishers {Geyx and Alcyone), by suppression of the inner front 

 toe ; and in the passerine genus Cholornis, by defect of the outer 

 front toe. Birds with two toes are said to be didadyl ; with three, 

 iridaetyl ; with four, tetradactyl. 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 osprey (Pandion). There is 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 (Caprimulgidce), some of the swifts (Cypselidce), the loons 

 (ColywMdce), and all the totipalmate swimmers (Steganopodes). The 

 rarest of all conditions (seen in some Cypselidce, and the African 

 CoUidce) is that in which all four toes are turned forward. The 

 arrangement of toes in pairs, two before and two behind, is quite 

 common, being characteristic of scansorial birds and some others, as 

 all the parrots and woodpeckers, cuckoos, trogons, etc. Such 

 arrangement is called zygodactyl or nygodadylous (Gr. fuyov, zugon, 

 a yoke ; Sa/cruAos, daktulos, a digit) ; and birds exhibiting it are said 



