Reptilia_.

The reptiles which exist in the world to-day may be classed in four
well-marked sets, each of which has the value of an "order"--(1)
crocodiles, (2) lizards, (3) serpents, and (4) tortoises. The names of
these creatures alone suffice to indicate the fact that the class of
reptiles presents us with an extraordinary amount of diversity of form
as compared with the class of birds with which, nevertheless, reptiles
have, as we shall hereafter see, very close relations. Indeed, in the
diversity of kinds which it contains, the class _Reptilia_ at the least
fully equals the class Mammalia, especially if the extinct kinds are
taken into consideration. The number of species of reptiles, both living
and extinct, much exceeds also the number of living and extinct mammals.

To begin once more with forms which are the least strange and unknown,
we may start with the little elegant and harmless lizards of our heaths
and commons, which will serve as types of the order to which they
belong--the order _Lacertilia_. That order is an extremely numerous one,
containing many families, differing much in form. Our English lizards
are true lizards, belonging to the typical genus _Lacerta_ and to the
typical family _Lacertidae_. The rather well-known large American lizard,
_Iguana_, is the type of another and very extensive family (almost
entirely confined to America), while a nearly-allied family (_Agamidae_)
is an Old World group. Amongst the curious forms found in the latter
family may be mentioned the frilled and moloch lizards of Australia, and
those little harmless lizards of India which go by the formidable name
of "flying dragons" (_Draco_). They are the only existing aerial
reptiles--not that they can truly "fly" at all, but they are enabled to
take prolonged jumps, and to sustain themselves to a considerable extent
in the air by means of the extremely distensible skin of their flanks
which, when extended, is supported by a peculiar solid framework
hereafter to be described. Some of the largest lizards are called
"monitors," and are common in Egypt; they belong to the family
_Monitoridae_.

In the warmest period of the year, certain lizards are found in the
South of Europe, called geckos. They have a power of running, not only
up walls, but across ceilings by means of a peculiar structure of their
toes. They are types of a large family (_Geckotidae_) widely spread over
the world.

Another large family (_Scincidae_) has