neck (fig. 176),
and probably possessing a simply smooth or wrinkled skin, since
no traces of scales or bony integumentary plates have ever been
discovered. The tail was long, and was probably furnished at its
extremity with a powerful expansion of the skin, constituting a
tail-fin similar to that possessed by the Whales. The limbs are
also like those of Whales in the essentials of their structure,
and in their being adapted to act as swimming-paddles. Unlike
the Whales, however, the Ichthyosaurs possessed the hind-limbs
as well as the fore-limbs, both pairs having the bones flattened
out and the fingers completely enclosed in the skin, the arm
and leg being at the same time greatly shortened. The limbs are
thus converted into efficient "flippers," adapting the animal
for an active existence in the sea. The different joints of the
backbone (vertebrae) also show the same adaptation to an aquatic
mode of life, being hollowed out at both ends, like the biconcave
vertebrae of Fishes. The spinal column in this way was endowed
with the flexibility necessary for an animal intended to pass
the greater part of its time in water. Though the _Ichthyosaurs_
are undoubtedly marine animals, there is, however, reason to
believe that they occasionally came on shore, as they possess
a strong bony arch, supporting the fore-limbs, such as would
permit of partial, if laborious, terrestrial progression. The
head is of enormous size, with greatly prolonged jaws, holding
numerous powerful conical teeth lodged in a common groove. The
nature of the dental apparatus is such as to leave no doubt as
to the rapacious and predatory habits of the Ichthyosaurs--an
inference which is further borne out by the examination of their
petrified droppings, which are known to geologists as "coprolites,"
and which contain numerous fragments of the bones and scales
of the Ganoid fishes which inhabited the same seas. The orbits
are of huge size; and as the eyeball was protected, like that
of birds, by a ring of bony plates in its outer coat, we even
know that the pupils of the eyes were of correspondingly large
dimensions. As these bony plates have the function of protecting
the eye from injury under sudden changes of pressure in the
surrounding medium, it has been inferred, with great probability,
that the Ichthyosaurs were in the habit of diving to considerable
depths in the sea. Some of the larger specimens of _Ichthyosaurus_
which have been discovered in t