276 VERTEBRATE ANIMALS. 
in which they agree with the flying birds. Many of the bones, 
namely, were “pneumatic’”—that is to say, were hollow, and 
were filled with air, thus giving the animal the degree of light- 
ness necessary for flight. Secondly, whilst the shoulder-girdle 
has many of the characters of birds, the breastbone (sternum) 
is furnished with a prominent ridge or keel, serving -for the 
attachment of the great muscles which work the wings. There 
can be no doubt, therefore, as to the Pterodactyles having 
enjoyed the power of genuine flight. Many of them attained 
no great size, but some of them must have been gigantic, the 
expanse of wing in one species having been calculated at pro- 
bably about twenty-seven feet from tip to tip. 
