254 TUE ANATOMY OF VERTEBRATED ANIMALS. 



outer, distally. In most birds, the middle metatarsal does not 

 remain parallel with the others, but its proximal end inclines 

 a little backward, and its distal end a little forward. Hence 

 the two apertures on each side of its proximal end may lie at 

 the bottom of a fossa, or run into one, in front, while they 

 remain distinct behind. 



Again, in most birds, the posterior face of the proximiil 

 end of the middle metatarsal, and the adjacent surface of the 

 tarsal bone, grow out into a process, which is commonly, 

 but improperly, termed " calcaneal." The inferior surface 

 of this hypo-tarsus is sometimes simply flattened, sometimes 

 traversed by grooves or canals, for the flexor tendons of the 

 digits. 



When a hallux exists, its metatarsal bone is usually in- 

 complete above, and is united by ligament to the inner, or the 

 jjosterior, surface of the tarso-metatarsus. In the Frigate-bird 

 {Phaethon) and in Steatornis, the hallucal metatarsal is re- 

 markably long. The genus Phaethon stands alone, so far as 

 I know, in having the hallucal metatarsal anchylosed with the 

 others. 



In many of the Alectoromorj^hoe, a spur (calcar), consisting 

 of a bony core ensheatlied in horn, is developed on the inner 

 side of the metatarsus, and becomes anchylosed with the meta- 

 tarsal of the second digit. In a few birds, similar spurs [Fala- 

 medea), or osseous excrescences (Pezojyhaps), are developed 

 in relation with the metacarpus. 



The normal number of the pedal phalanges in birds is (as 

 in ordinary Lacertilia) 2, 3, 4, 5, reckoning from the hallux to 

 the fourth digit. Among the few birds which constitute ex- 

 ceptions to the rule are the Swifts, in which the third and 

 fourth toe have only three phalanges each (3, 3, 3, 3), and the 

 Goatsuckers and the Sand-grouse, in which the fourth toe 

 only has the number thus reduced (2, 3, 4, 3). 



Many birds have only three toes, by suppression of the 

 hallux. In the Ostrich, not only the hallux, but the phalanges 

 of the second digit, are suppressed; and the distal end of the 

 second metatarsal is reduced to a mere rudiment. Hence the 

 ostrich has only two toes (which answer to the third and fourth 

 of the pentadactyle foot), with four phalanges in the inner and 

 five in the outer, though the inner toe is far the longer and 

 the stronger. 



In most four-toed birds the hallux is turned more or less 

 completely backward, and the other tliree digits forward. But 

 in many Aetomorphce (especially the Owls), the outer toe can 



