these bat-like creatures we rise until we come to such
dragons as the American Pteranodon, with a stretch of twenty-two feet
between its extended wings and jaws about four feet long. There were
long-tailed Pterosaurs (Ramphorhyncus), sometimes with a rudder-like
expansion of the end of the tail, and short-tailed Pterosaurs
(Pterodactyl), with compact bodies and keeled breasts, like the bird. In
the earlier part of the period they all have the heavy jaws and numerous
teeth of the reptile, with four or five well-developed fingers on the
front limbs. In the course of time they lose the teeth--an advantage
in the distribution of the weight of the body while flying--and develop
horny beaks. In the gradual shaping of the breast-bone and head, also,
they illustrate the evolution of the bird-form.

But the birds were meantime developing from a quite different stock,
and would replace the Pterosaurs at the first change in the environment.
There is ground for thinking that these flying reptiles were
warm-blooded like the birds. Their hollow bones seem to point to the
effective breathing of a warm-blooded animal, and the great vitality
they would need in flying points toward the same conclusion. Their
brain, too, approached that of the bird, and was much superior to that
of the other reptiles. But they had no warm coats to retain their heat,
no clavicle to give strength to the wing machinery, and, especially in
the later period, they became very weak in the hind limbs (and therefore
weak or slow in starting their flight). The coming selection will
therefore dismiss them from the scene, with the Deinosaurs and
Ammonites, and retain the better organised bird as the lord of the air.

There remain one or two groups of the Mesozoic reptiles which are still
represented in nature. The turtle-group (Chelonia) makes its appearance
in the Triassic and thrives in the Jurassic. Its members are extinct and
primitive forms of the thick-shelled reptiles, but true turtles, both
of marine and fresh water, abound before the close of the Mesozoic.
The sea-turtles attain an enormous size. Archelon, one of the primitive
types, measured about twelve feet across the shell. Another was thirteen
feet long and fifteen feet from one outstretched flipper to the other.
In the Chalk period they form more than a third of the reptile remains
in some regions. They are extremely interesting in that they show, to
some extent, the evolution of their characteristic