The optic lobes are two, instead of being corpora quadrigemina,
and hollow.

In the hind-brain--

The cerebellum is a very small transverse band, and has no
lateral parts.

The medulla is relatively larger.

There are no spinal accessory nor hypoglossal nerves to the
brain.

2. Compare the vertebrae of dog-fish, rabbit, and frog.

The centra of the dog-fish are -opistho- [amphi]-coelous
(i.e., hollow at either end).

The centra of the rabbit are flat-faced.

The centra of the frog are procoelous (hollow in front).

The notochord persists between the centra in the dog-fish and
rabbit, within the centra in frog.

The centra of the rabbit have epiphyses, absent in the dogfish
and frog.

The transverse processes of the rabbit typically bear ribs.
Short ribs occur in the dog-fish, but their homology with those
of the rabbit is doubtful. The frog has no ribs.

The interneural plates are peculiar to the dog-fish in this
comparison.

3. Compare the skull of the dog with that of the frog.

The Brain Case--

Of the frog is a cylindrical box, from which the otic capsules
project conspicuously on either side. It contains only two
ossifications in its cartilaginous substance (the sphen-ethmoid
and the ex-occipital), being protected by the membrane bones,
the parieto-frontals above and the parasphenoid below.

In the mammal it is enormously inflated, and the otic capsules
are imbedded in its wall. There are supra- and basi- as well as
ex-occipital bones; the para-sphenoid is (? entirely) gone, and
its place is taken by the basi- and pre-sphenoids, and the
lateral walls contain fresh paired ossifications, the ali- and
orbito-sphenoids-- all cartilage bones. The sphenethmoid is
perhaps represented in part by the ethmoid.

As a result of the inflation of the brain-case, the squamosal,
which slopes downward and outward in the frog, and overlies the
cartilaginous suspensorium (quadrate cartilage), has become a
constituent of the brain-case wall, and slopes downwardly and in.

Jaw Suspension--

The point of attachment of the jaw has shifted outward, and the
original suspensorial cartilage (the quadrate) has taken on a new
and minor function as the incus of the middle ear-- the squamosal
superseding it as the