shape.

The earliest amphibians differ from those of to-day in a number of
respects. They were connecting types linking together fishes, from
which they were descended, with reptiles, of which they were the
ancestors. They retained the evidence of their close relationship
with the Devonian fishes in their cold blood, their gills and
aquatic habit during their larval stage, their teeth with dentine
infolded like those of the Devonian ganoids but still more
intricately, and their biconcave vertebrae which never completely
ossified. These, the highest vertebrates of the time, had not yet
advanced beyond the embryonic stage of the more or less
cartilaginous skeleton and the persistent notochord.

On the other hand, the skull of the Carboniferous amphibians was
made of close-set bony plates, like the skull of the reptile,
rather than like that of the frog, with its open spaces (Figs. 313
and 314). Unlike modern amphibians, with their slimy skin, the
Carboniferous amphibians wore an armor of bony scales over the
ventral surface and sometimes over the back as well.

It is interesting to notice from the footprints and skeletons of
these earliest-known vertebrates of the land what was the
primitive number of digits. The Carboniferous amphibians had five-
toed feet, the primitive type of foot, from which their
descendants of higher orders, with a smaller number of digits,
have diverged.

The Carboniferous was the age of lycopods and amphibians, as the
Devonian had been the age of rhizocarps and fishes.