These are primitive jaw-bearing fishes, the majority of which were
heavily armored (Pl. 37). The _placoderms_ were shark-like in
appearance, and some of them grew to be as much as 30 feet in length.
Members of this class appeared first in the Devonian and lasted into the
Permian, at which time they became extinct. Placoderms are rare in
Texas, but the fragmentary remains of these primitive fishes have been
found in Devonian rocks in central Texas.