A REPTILE IS KNOWN BY ITS BONES
Such are a few illustrations of ways in which reptiles resemble other
animals, and differ from them, in the organs by means of which the
classification of animals is made. But such an idea is incomplete
without noticing that the bony framework of the body associated with
such vital organs also shows in its chief parts that reptiles are easily
recognised by their bones. I will therefore briefly state how reptiles
are defined in some regions of the skeleton, for in tracing the history
of reptile life the bones are the principal remains of animals preserved
in the rocks; and the soft organs which have perished can only be
inferred to have been present from the persistence of durable
characteristic parts of the skeleton, which are associated with those
soft organs in animals which exist at the present day, and are unknown
in other animals in which the skeleton is different.
THE HANG OF THE LOWER JAW
The manner in which the lower jaw is connected with the skull yields one
of the most easily recognised differences between the great groups of
vertebrate animals.
_In Mammals._--In every mammal--such as the Dog or Sheep--the lower jaw,
which is formed of one bone on each side, joins directly on to the head
of the animal, and moves upon a bone of the skull which is named the
temporal bone. This character is sufficient to prove, by the law of
association of soft and hard parts of the body, that such an animal had
warm blood and suckled its young.
[Illustration: FIG. 2 _PTERODACTYLUS KOCHI_ SKULL OF BEAR
Comparison to show the articulation with the lower jaw in a mammal
and _Pterodactylus Kochi_. The quadrate bone is lettered Q in this
Pterodactyle, and comes between the skull and the lower jaw like the
quadrate bone in a bird and in lizards.]
_In Birds._--In birds a great difference is found in this region of the
head. The temporal bone, which it will be more convenient to name the
squamosal bone, from its squamous or scale-like form, is still a part of
the brain case, and assists in covering the brain itself, exactly as
among mammals. But the lower jaw is now made up of five or six bones.
And between the hindermost and the squamosal there is an intervening bar
of bone, unknown among mammalia, which moves upon the skull by a joint,
just as the lower jaw moves upon it. This movable bone unites with parts
of the palate and the face, and is known as the quadrate bone. Its
presence proves that the animal possessing it laid eggs, and if the
face bones join its outer border just above the lower jaw, it proves
that the animal possessed hot blood.
_In Reptiles._--All reptiles are also regarded as possessing the
quadrate bone. But the squamosal bone with which it always unites is in
less close union with the brain case, and never covers the brain itself.
Serpents show an extreme divergence in this condition from birds, for
the squamosal bone appears to be a loose external plate of bone which
rests upon the compact brain case and gives attachment to the quadrate
bone which is as free as in a bird. Among Lizards the quadrate bone is
usually almost as free. In the other division of existing Reptilia,
including Crocodiles, the New Zealand lizard-like reptile Hatteria,
called Tuatera, and Turtles, the squamosal and quadrate bones are firmly
united with the bones of the brain case, face, and palate, so that the
quadrate bone has no movement; and the same condition appears in
amphibians, such as Toads and Frogs. With these conditions of the
quadrate bone are associated cold blood, terrestrial life, and young
developed from eggs.
_In Fishes._--Bony fishes, and all others in which separate bones build
up the skull, differ from Reptiles and Birds much as those animals
differ from Mammals. The union of the lower jaw with the skull becomes
complicated by the presence of additional bones. The quadrate bone still
forms a pulley articulation upon which the lower jaw works, but between
it and the squamosal bone is the characteristic bone of the fish known
as the hyomandibular, commonly connected with opercular bones and
metapterygoid which intervene, and help to unite the quadrate with the
brain case. In the Cartilaginous fishes there is only one bone
connecting the jaws with the skull on each side. This appears to prove
that just as the structure of the arch of bones suspending the jaw may
be complicated by the mysterious process called segmentation, which
separates a bone into portions, so simplification and variation may
result because the primitive divisions of the material cease to be made
which exists before bones are formed.
The principal regions of the skull and skeleton all vary in the chief
groups of animals with backbones; so that the Reptile may be recognised
among fossils, even in extinct groups of animals and occasionally
restored from a fragment, to the aspect which characterised it while it
lived.