THE PEDIGREE OF BIRDS 347 



§ 2. Affiliation of Birds to Reptiles 



A bird is known by its feathers, and no transition is known 

 between feathers and any other integumentary structure, 

 yet we have only to look at the tarsometatarsus (ankle-and- 

 instep region) of an ordinary bird to see that the legacy of 

 reptilian scales is not lost. There is no doubt, moreover, 

 that the horny covering of a bird's beak is comparable to the 

 scales of reptiles, especially when we notice that in old- 

 fashioned birds like the albatross and the petrel it is com- 

 pound, that is to say built up of many separate pieces. An 

 interesting corroboration is to be found in the fact that the 

 puffin moults the outer covering of its bill-scales every year, 

 a process much nearer to the moulting seen in reptiles (where 

 the outer layer of the epidermis covering the scales is peeled 

 off) than to the moulting seen in birds (where individual 

 feathers fall off as such). Another hint of affinity may 

 perhaps be found in the little hardening of horn and lime 

 formed at the tip of the bill of many birds before they are 

 hatched, and somewhat unfortunately called the egg-tooth. 

 For a similar structure, apparently of use in breaking out of 

 the egg-shell, is found in some unhatched reptiles. It 

 appears in birds even before the laying down of the horny 

 bill, and the suggestion has been made that it is the last relic 

 of a very ancient armature, older than ordinary scales. 



There is often a claw on the thumb of a bird's wing, and 

 sometimes, as in the ostrich, on the first finger as well. In 

 the young Hoatzin (Opisthocomus) the claws on the thumbs 

 are unusually long, and are mobile enough to be used in 

 scrambling. In Archasopteryx there was a claw on each of 

 the three digits. Here again, therefore, there are relics of 

 reptilian ancestry. 



The skull of a bird is very different from that of any 

 reptile, and yet the two types agree in having one occipital 

 condyle for working against the first vertebra, a complex 

 mandible or lower jaw made up of 4-6 bones on each side, 

 an articulation of the lower jaw with the quadrate in the 



