e. We turn from them to the remaining
orders of the Jurassic reptiles.

In the popular mind, perhaps, the Ichthyosaur and Plesiosaur are the
typical representatives of that extinct race. The two animals, however,
belong to very different branches of the reptile world, and are by no
means the most formidable of the Mesozoic reptiles. Many orders of the
land reptiles sent a branch into the waters in an age which, we saw, was
predominantly one of water-surface. The Ichthyosauria ("fish-reptiles")
and Thalattosauria ("sea-reptiles") invaded the waters at their first
expansion in the later Triassic. The latter groups soon became extinct,
but the former continued for some millions of years, and became
remarkably adapted to marine life, like the whale at a later period.

The Ichthyosaur of the Jurassic is a remarkably fish-like animal. Its
long tapering frame--sometimes forty feet in length, but generally less
than half that length--ends in a dip of the vertebral column and an
expansion of the flesh into a strong tail-fin. The terminal bones of the
limbs depart more and more from the quadruped type, until at last they
are merely rows of circular bony plates embedded in the broad paddle
into which the limb has been converted. The head is drawn out, sometimes
to a length of five feet, and the long narrow jaws are set with two
formidable rows of teeth; one specimen has about two hundred teeth. In
some genera the teeth degenerate in the course of time, but this
merely indicates a change of diet. One fossilised Ichthyosaur of the
weaker-toothed variety has been found with the remains of two hundred
Belemnites in its stomach. It is a flash of light on the fierce struggle
and carnage which some recent writers have vainly striven to attenuate.
The eyes, again, which may in the larger animals be fifteen inches in
diameter, are protected by a circle of radiating bony plates. In fine,
the discovery of young developed skeletons inside the adult frames has
taught us that the Ichthgosaur had become viviparous, like the mammal.
Cutting its last connection with the land, on which it originated it
ceased to lay eggs, and developed the young within its body.

The Ichthyosaur came of the reptile group which we have called the
Diapsids. The Plesiosaur seems to belong to the Synapsid branch. In the
earlier Mesozoic we find partially aquatic representatives of the line,
like the Nothosaur, and in the later Plesiosaur the adaptation to a
marine li