ch, unlike the corresponding organ of the air-frequenting vertebrata,
has no internal narial opening. There is, however, a groove running
from olf. to the corner of the mouth, and this, closing, in the vertebrate
types that live in air and are exposed to incessant evaporation of their
lubricating secretions, constitutes the primitive nasal passage. The
limbs are undifferentiated into upper, lower, and digital portions, and
are simply jointed, flattened expansions.

Section 3. The skin of the dog-fish is closely set with pointed
tooth-like scales, the placoid scales, and these are continued over
the lips into the mouth as teeth. Each scale consists of a base of true
bone, with a little tubercle of a harder substance, dentine, capped by
a still denser covering, the enamel. The enamel is derived from the
outer layer of the embryonic dog-fish, the epiblast, which also gives
rise to the epidermis; while the dentine and bony base arise in the
underlying mesoblast, the dermis. A mammalian tooth has
essentially the same structure: an outer coat of enamel, derived from
epiblast, overlies a mass of dentine, resting on bone, but the dentine
is excavated internally, to form a pulpcavity containing blood-vessels
and nerves. Most land animals, however, have teeth only in their
mouths, and have lost altogether the external teeth which constitute
the armour of the dog-fish. Besides the teeth there perhaps remain
relics of the placoid scales in the anatomy of the higher vertebrata, in
the membrane bones. How placoid scales may have given rise to
these structures will be understood by considering such a bone as
the vomer of the frog. This bone lies on the roof of the frog's mouth,
and bears a number of denticles, and altogether there is a very strong
resemblance in it to a number of placoid scales the bony bases of
which have become confluent. In the salamander, behind the
teeth-bearing vomers comes a similar toothed parasphenoid bone.
The same bone occurs in a corresponding position in the frog, but
without teeth. In some tailed amphibians the vomers and splenials are
known to arise by the fusion of small denticles. These facts seem to
point to stages in the fusion of placoid bases, and their withdrawal
from the surface to become incorporated with the cranial apparatus as
membrane bones, a process entirely completed in the mammalian
type.

Section 4. The alimentary canal of the dog-fish, is a simple tube
thrown into a Z shape. The