FLIGHTLESS BIRDS. 9 
the head of the compound bone, considered to be the 
metacarpal bone of the thumb or pollex; the second bone 
being much the largest and broadest of the three component 
parts, corresponding to the index finger; whilst the third 
seement is nearly as long as the second, but is a much 
slenderer bone than this, with which it is fused at its two 
extremities, corresponding with the third digit. At the 
end of this triply-compound metacarpal bone come the 
phalanges forming the digits, represented in the Duck, as 
in most birds, by two for the first metacarpal, two for the 
second, the proximal phalanx of the second digit being 
much the largest and broadest in correspondence with the 
large metacarpal from which it arises, and one for the 
third. Though this triply-compound metacarpal bone, 
with the three digits at the end, is supposed to correspond 
with the thumb and first two digits of the generalised 
vertebrate type, some authorities consider that it is more 
correct to regard them as the second, third, and fourth 
digits, the thumb being suppressed altogether. 
-_ Now this peculiar arrangement of metacarpals and 
phalanges is remarkably constant and characteristic of ail 
modern flying birds, and it may be seen equally well- 
developed in the strong flying Ichthyornts of the Cretaceous 
period. It is a type specially developed, so as to allow 
little individual motion in the digits, and, as such, it is 
highly characteristic of flying birds, where compactness 
and immobility in these parts are required. It would be 
very unlikely that such a highly specialised type of manus 
would be developed independently for a different purpose, 
and if the wing of the Ostrich had never been used for the 
purpose of flight, we should not expect to find in its 
manus the type characteristic of flying birds. Yet this is 
just what occurs. We find the same triply-compound 
metacarpal bone, and the same arrangement of the 
