This class includes such modern forms as the sharks, rays, and skates.
They are characterized by skeletons which are composed of cartilage and
are very abundant in the marine waters of today. The earliest known
representatives of this class are reported from rocks of Devonian age,
and they have been relatively common up to the present time.
Shark teeth (Pl. 37) can be found in Texas in Pennsylvanian, Permian,
Cretaceous, Paleocene, Eocene, and Miocene rocks. These are probably the
most common vertebrate fossils to be found in Texas and are usually
found in thin-bedded marine limestones or clays.