442 ON THE STRUCTURE OF THE FOOT IN BIRDS, 



second digit, on the other hand, is reduced to its basal phalanx (fig. 1, iv), 

 thus appearing externally merely as a wart-like eminence on the side of 

 the digit next to it, in a way very similar to that exhibited by some 

 Edentata, in which the fifth digit of the manus is greatly reduced. 



In the curious Passerine genus Cholornis, on the other hand, which is 

 also said to be three-toed, the reduction is brought about by the absorption 

 of the most external, or fourth, digit (fig. 1, v) *. 



In the Ostrich, finally (fig. 1, vi), only two digits are present, both 

 the first and second having entirely disappeared. 



LIST OF TRIDACTYLE FAMILIES AND G-ENERA OF BIRDS. 

 A. By suppression of the hallux. 



Rheidae. Turnicidge (excl. Pedionomus). 



Casuariidae. Syrrhaptes [Pteroclidse]. 



Calodromas\ [Tinamidae]. Alcidae. 



Tinamotis t ,, Charadriidae (many genera, e. g. Charadrius, 



Ibis, 1882, Pelecanoides [Procellariidae]J. Hcematopus, Hoplopterus, Calidris). 



p. 389. Phoenicoparrus [Phcenicopteridaa]. Eissa (at least generally ; cf. Saunders, 



Otididse. P. Z. S. 1878, pp. 162, 163) [Laridse]. 



GEdicnemididae (incl. Esacus). Saaia, Picoides, Tiga [Picidse]. 



Cursorius. Jacamaralcyon [Galbulidse]. 



B. By suppression of the 2nd digit. 



Ceyx, Alcyone [Alcedinidee]. 



C. By suppression of the 4th digit. 



Cholornis [0 seines]. 



II. The Number of Phalanges. 



The normal number of phalanges in birds is, as already stated, 2, 3, 4, 



5, in the respective digits, counting from within outwards (fig. 2, i). In 



all the Tubinares , so far as I have seen, except Pelecanoides (where the 



Ibis, 1882, hallux is quite absent), the number of joints in the hallux is reduced to 



p. 6vO. one ^ w hi c h j s q u ite short, and covered by the spur-like claw ; the digital 



formula therefore becomes 1, 3, 4, 5 (fig. 2, n). 



* I have not myself yet had an opportunity of examining Cholornis paradoxa ; my 

 authorities for the statement here made are MM. David and Oustalet (Oiseaux de la 

 Chine, p. 205), who describe this bird as having the external digit reduced to a 

 " simple moignon." It would be interesting to know how far the reduction here has 

 progressed. 



t Sundevall places these two genera, with some doubt, amongst his " Otidinse " 

 (Tentamen, p. 128). A skull extracted from a skin of Calodromas in my possession 

 shows, however, that it, at least, is undoubtedly a Tinamou, the palate being perfectly 

 " dromseognathous." 



| The DiomedeincB, often described as three-toed, have a very minute and 

 rudimentary hallux (cf. P.Z. S. June 1882). 



Cf. Coues " On the Osteology of Colymbus torquatus " (Mem. Bost. Soc. Nat. 

 Hist. i. p. 161, note). 



