390 On the Structure of the Foot in Birds. 



absent), tlie number of joints in the balhix is reduced to one, 

 wliich is quite short, and covered by the spur-like claw ; the 

 digital foruiula therefore becomes 1, 3, 4, 5 (fig. 2, ii). 



In the true Swifts [i. e. the genera Cypselus and Pany- 

 ptila) "^j though not in the rest of the Cypselidse, the number of 

 phalanges in each digit external to the hallux is three, the 

 formula thus becoming 2, 3, 3, 3 (fig. 2, in). In the other 

 genera of Swifts the normal number of phalanges obtains, 

 as already correctly stated by Mr. Sclaterf. 



In the Pteroclidre and true Caprimulgidse, finally, the 

 formula is 2, 3, 4, 4 (not 2, 3, 4, 3, as stated by Prof. 

 Huxley J), the fourth digit being one short of the normal 

 number of phalanges (fig. 2, iv). The anomalous genera 

 Steatornis, jEgotheles, Nyctibius, &c. are normal as regards 

 the structure of their feet§, as also are Thinocorus and 

 Attogis. 



Nitzsch, who must have been misled by a badly articulated 

 skeleton, ascribes four joints each to all the toes, both of 

 Struthio and Casuarius\\. He was in doubt therefore as to 

 which digits were represented in these forms. As a fact, 

 I need scarcely remark, the normal number of phalanges 

 (4, 5 and 3, 4, 5 respectively) is present in both these 

 birds, though in museum specimens a joint or two is often 

 missing. In many Ostriches, however, the nail of the outer 

 toe is quite absent, and in others very small ; so that their 

 foot is evidently tending to become, like that of the Solipeds, 

 reduced to a single toe, in this as in other cases the third. 



* Nitzscli, so loiif^ ago as 1811, pointed out tliis fact, as well as the 

 reduction in Caprimulgus ("Ueber die Gliederung der Fusszeliou, be- 

 sonders im Zieg-enmelker und in der Mauerscbwalbe," Osteogr. Beitr. 

 pp. lOJ-lOo). 



t P. Z. S. 18G.5, p. 596.' 



X Possibly misled by an error in the figure of the skeleton of Si/rrhapfes 

 in Prof. Parker's memoir " On the Osteology of the Gallinaceous Birds 

 and Tinamous " (Tr. Z. S. v. pi. 38), where the outer toe is represented as 

 consisting of three joints only, though in the text (p. 20.3) the correct 

 number is accurately stated. 



§ Cf. Sclater, P. Z. S. 1866, p. 124. 



11 Oo. cit. p. 102. 



