XII PHYLUM CHORDATA 487 



hand of the adult bird has lost all the characters of a fore- 

 foot ; but in the young of the hoatzin ( Opisthdcomus) claws 

 are present on the first two digits (Fig. 295, A), which are 

 sufficiently mobile to be used in climbing. Besides the 

 true claws, horny spurs are sometimes present on the carpus 

 and metacarpus. 



There is almost every gradation in the proportional 

 length of the hind-limb, from birds in which nothing but 

 the foot projects beyond the contour feathers, and even the 

 toes may be feathered, to the long-legged storks and cranes, 

 in which the distal part of the tibio-tarsus is covered with 

 scales as well as the foot. In aquatic forms a fold of skin 

 or web is stretched between the toes, sometimes including 

 all four digits, as in the cormorants ; sometimes leaving the 

 hallux free, sometimes forming a separate fringe to each 

 digit, as in the coots and grebes. As to the toes them- 

 selves, the commonest arrangement is for the hallux to be 

 directed backwards, and Nos. 2, 3, and 4 forwards, but in 

 the owls No. 4 is reversible, i.e. can be turned in either 

 direction, and in the parrots, woodpeckers, etc., it, as well 

 as the hallux, is permanently turned backwards. In the 

 swifts, on the other hand, all four toes turn forwards. The 

 hallux is frequently vestigial or absent, and in the ostrich 

 No. 4 has also atrophied, producing the characteristic two- 

 toed foot of that bird. 



The following are the most essential general features of the 

 skeleton of birds. More or fewer vertebrae from the regions 

 in front of and behind the sacral fuse with the true sacral 

 vertebras to form the composite syn-sacrum. The posterior 

 caudal vertebrae are fused together to form a pygostyle. 

 The bones of the skull early unite, the sutures becoming 

 entirely obliterated. There is a single rounded occipital 

 condyle. The premaxillas are very large and form the 



