These are typical star-shaped free-moving echinoderms in which the body is divided into a central disk and radiating arms.
Subclass Asteroidea.—
This class contains the starfishes which, although not common fossils, illustrate well the typical [echinoderm] characteristics ([Pl. 35]). [Fossil] starfishes have been found sparingly in certain formations in Texas, but well-preserved specimens are quite rare. However, excellently preserved starfishes have been found in slabs of [Cretaceous] limestones from central and north-central Texas.
Subclass Ophiuroidea.—
The ophiuroids are echinoderms with a well-defined central disk and five long, slender, whip-like arms. They have been called brittle stars because of their ability to shed their arms when they are disturbed. Their long, slender, snake-like arms have also resulted in their being called serpent stars. Ophiuroids range from [Ordovician] to Recent in age, but because of the delicate nature of their bodies they are seldom found as fossils. Ophiuroid remains have been found in certain [Mesozoic] and [Cenozoic] rocks of Texas, but they consist largely of small segments of the arms or body fragments.
CLASS ECHINOZOA.—
The echinozoans are a group of unattached echinoderms whose bodies consist of numerous [calcareous] plates and spines, but they do not possess the radiating arm-like extensions which characterize the asterozoans.
Subclass Echinoidea.—
Echinoids are free-moving echinoderms with disk-shaped, heart-shaped, biscuit-shaped, or globular exoskeletons ([Pl. 36]). Modern representatives of this group include the familiar sea urchins, heart urchins, and the sand dollars.
Plate 35