The _Jurassic_ period showed a continuance of the reptilian splendour.
They radiated in many directions, becoming adapted to many haunts. Thus
there were many Fish Lizards paddling in the seas, many types of
terrestrial dragons stalking about on land, many swiftly gliding
alligator-like forms, and the Flying Dragons which began in the Triassic
attained to remarkable success and variety. Their wing was formed by the
extension of a great fold of skin on the enormously elongated outermost
finger, and they varied from the size of a sparrow to a spread of over
five feet. A soldering of the dorsal vertebræ as in our Flying Birds was
an adaptation to striking the air with some force, but as there is not
more than a slight keel, if any, on the breast-bone, it is unlikely that
they could fly far. For we know from our modern birds that the power of
flight may be to some extent gauged from the degree of development of
the keel, which is simply a great ridge for the better insertion of the
muscles of flight. It is absent, of course, in the Running Birds, like
the ostrich, and it has degenerated in an interesting way in the
burrowing parrot (_Stringops_) and a few other birds that have "gone
back."