Fishes belonging to this class are primitive, jawless, and represented
by the living lampreys and hagfishes. The first agnathans appeared in
the Ordovician and were armored by a bony covering on the front part of
their bodies. These primitive fishes, called _ostracoderms_, are the
earliest recorded fishes and, in addition, appear to be the first known
vertebrate animals. The ostracoderms first appeared in late Ordovician
time, increased in numbers in the Silurian, and were extinct by the end
of the Devonian.