Articulate brachiopods have a well-defined hinge-line (fig. 13a). One
valve has well-developed teeth which articulate with sockets in the
opposing valve, and there is a well-developed muscle system which aids
in opening and closing the shell. Articulate brachiopods are
characterized by calcareous shells which are typically of unequal size
and a wide variety of shapes (Pls. 18, 19). The class has been divided
into several orders which have been established primarily on the nature
of the pedicle foramen and the nature of shell growth.
Articulate brachiopods range from Lower Cambrian to Recent in age and
are particularly abundant in certain Pennsylvanian formations of
north-central and Trans-Pecos Texas. They are also present in certain
other fossiliferous strata of Paleozoic age but are less abundant and
not as well preserved. The only Cretaceous brachiopod that is found in
large numbers is _Kingena wacoensis_ (Roemer) (fig. 15), which is
particularly abundant in certain formations in the upper part of the
Comanche series.
[Illustration: Fig. 15. _Kingena wacoensis_, a common Cretaceous
brachiopod. (a) Dorsal view. (b) Lateral view. (c) Ventral view.]