EXTERNAL FEATURES 23 



almost always primitive, less specialised than the down 

 feathers of the adult if it has any. Thus in most cases the 

 calamus is very short, the rhachis is absent or ill-defined, 

 the barbs are long and slender, the barbules are numerous, 

 the barbicels are absent. There is no aftershaft except in the 

 Emeu. Penguins are remarkable, since the minute structure 

 of the adult down feathers is the same as that of the young, 

 which resembles the nestling down of ducks, rails, and 

 cormorants. This hints at primitiveness in penguins. 

 In ostriches the nestling feathers show their highest develop- 

 ment ; thus the calamus is well-formed and the barbs of 

 the back feathers are terminally expanded and flattened, 

 giving the chick a coarse bristly appearance. 



Furthermore, it is usual to distinguish two sets or 

 generations of prepennae — the first set called protoptiles 

 and the second set called mesoptiles. The penguins show 

 the two sets very clearly, but in most birds there is only 

 one nestling coat, and it is a difficult question to decide 

 whether this represents the first or the second set. The 

 protoptiles or the mesoptiles may be vestigial or even 

 suppressed. 



The adult pennse may be divided into plumose and 

 pennaceous types. The plumose pennae, not to be confused 

 with adult down feathers, have no booklets or hamuli on 

 their barbules ; the pennaceous pennae have some of their 

 barbules equipped with booklets binding the barbs into a 

 coherent web. 



The following scheme, which we have ventured to 

 invert, is suggested by Professor Cossar Ewart (1921) : — 



Filoplumes Plumose Pennae Pennaceous Penn^ Plumulae 



or or 



Metaptiles Teleoptiles 



II. Prepennae 

 (Mesoptiles) 



Prefiloplumes I. Prepennas Preplumulas 



(Protoptiles) 



I 



Filaments or Cryptoptiles 



I. 

 Papillae 



