he Lias indicate an animal of
from 20 to nearly 40 feet in length; and many species are known
to have existed, whilst fragmentary remains of their skeletons
are very abundant in some localities. We may therefore safely
conclude that these colossal Reptiles were amongst the most
formidable of the many tyrants of the Jurassic seas.

[Illustration: Fig. 177.--_Plesiosaurus dolichodeirus_, restored.
Lias.]

The _Plesiosaurus_ (fig. 177) is another famous Oolitic Reptile,
and, like the preceding, must have lived mainly or exclusively in
the sea. It agrees with the Ichthyosaur in some important features
of its organisation, especially in the fact that both pairs of
limbs are converted into "flippers" or swimming-paddles, whilst
the skin seems to have been equally destitute of any scaly or bony
investiture. Unlike the _Ichthyosaur_, however, the Plesiosaur
had the paddles placed far back, the tail being extremely short,
and the neck greatly lengthened out, and composed of from twenty
to forty vertebrae. The bodies of the vertebrae, also, are not
deeply biconcave, but are flat, or only slightly cupped. The
head is of relatively small size, with smaller orbits than those
of the _Ichthyosaur_, and with a snout less elongated. The jaws,
however, were armed with numerous conical teeth, inserted in
distinct sockets. As regards the habits of the Plesiosaur, Dr
Conybeare arrives at the following conclusions: "That it was
aquatic is evident from the form of its paddles; that it was
marine is almost equally so from the remains with which it is
universally associated; that it may have occasionally visited
the shore, the resemblance of its extremities to those of the
Turtles may lead us to conjecture: its movements, however, must
have been very awkward on land; and its long neck must have impeded
its progress through the water, presenting a strong contrast to
the organisation which so admirably fits the _Ichthyosaurus_
to cut through the waves." As its respiratory organs were such
that it must of necessity have required to obtain air frequently,
we may conclude "that it swam upon or near the surface, arching
back its long neck like a swan, and occasionally darting it down
at the fish which happened to float within its reach. It may
perhaps have lurked in shoal water along the coast, concealed
amongst the sea-weed; and raising its nostrils to a level with
the surface from a considerable depth, may have found a secure
retreat from the