The members of this class are rather primitive and have a long geologic history. These brachiopods have valves which are not provided with hinge teeth, the valves being held together by muscles, and a hinge-line is lacking (fig. 14). Most inarticulate brachiopods are circular or tongue-like in shape and commonly composed of chitinous and phosphatic material. Inarticulate brachiopods range from Lower Cambrian to Recent in age but were never as common as the articulate brachiopods, which are described below. Brachiopods belonging to this class have been recorded from several Paleozoic formations in Texas (Pl. 17, _Lingula_, _Apsotreta_, _Angulotreta_). [Illustration: Plate 17] [Illustration: PENNSYLVANIAN BRYOZOANS] FISTULIPORA × 6 POLYPORA × 5 RHOMBOPORA × 8 [Illustration: CAMBRIAN BRACHIOPODS] APSOTRETA × 10 LINGULA × 4 ANGULOTRETA × 10 [Illustration: MISSISSIPPIAN BRACHIOPODS] RHIPODOMELLA × 1 DICTYOCLOSTUS × 1 CAMAROTOECHIA × 1 [Illustration: Fig. 14. _Lingula_, a typical Recent inarticulate brachiopod showing extended pedicle.] Pedicle Valve CLASS ARTICULATA.— 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.] Phylum Mollusca