388 Mr. W. A. Forbes on the Structure 



Picoides*) and Jacamars {Jacamaralcyon) f. It is not 

 always, however, the hallux that is thus absent in tridactyle 

 birds. In the Kingfishers of the genera Ceyx and Alcyone 

 the foot is three-toed, but the hallux is well developed ; the 

 second digit, on the other hand, is reduced to its basal pha- 

 lanx (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 raanus 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)J. 



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

 present, both the first and second having entirely disappeared. 



List of Tbidactyle Families and Genera of Birds. 



A. By suppression of the hallux. 



Rheidse. Calodromas § [Tinamiclce ]. 



Casuariidte. Tinamotis § „ 



* By some error Nitzsch (Osteograph. Beitr. p. 102) describes Picoides 

 as lacking the fourth (" letzte ") toe. As I have lately shown, however 

 (P. Z. S. June 1882), there is a rudimentary hallux, with its metatarsal, 

 in these birds, though it is quite concealed under the skin, and has, in 

 consequence, been overlooked by previous observers. The existence of 

 a similarly concealed rudimentary hallux in many other birds apparently 

 tridactyle is therefore rendered highly probable. 



t The specific name of Loxia tridadyla (Gmel. Syst. Nat. i. p. 866; 

 Phytotoma tridactyla, Daud. Tr. Orn. ii. p. 366) seems to be a mistake, 

 founded on Bruce's drawing of a bird met with by him in Abyssinia, and 

 mentioned by Buffon (Hist. Nat. Ois. iii. p. 471) under the name of "Le 

 Guifso Balito." This is usually identified as a well-known Abyssinian 

 Barbet {rogonorhynchus abyssiniciis, Marshall, Mon. Capit. pi. 9), with 

 feet of the normal structure. 



\ T have not myself yet had an opportunity of examining Cholornis 

 pnrado.va ; 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. 



§ Sundevall places these two genera, with some doubt, amongst his 



