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.
[Illustration: Plate 35]
CRINOIDS
CRINOID CALYX × ½
CRINOID COLUMNALS × 1
HOLOTHURIAN SCLERITES (GREATLY ENLARGED)
CRETACEOUS FOSSIL STARFISHES
PENTAGONASTER × 1
PENTACEROS × 1
[Illustration: Plate 36
CRETACEOUS ECHINOIDS]
SALENIA × 1
ECHINOID SPINES × 2
ECHINOID PLATE × 2
HEMIASTER × 1
HOLASTER × 1
HOLECTYPUS × 1
The echinoid _test_ (exoskeleton) is composed of many intricately
fitting calcareous plates (Pl. 36) which enclose the animal’s soft
parts. The exterior of the test is typically covered with large numbers
of movable spines (Pl. 36) which vary greatly in size. These spines are
of some aid in locomotion, support the skeleton of the animal, and
provide a measure of protection from enemies.
The oldest known echinoids have been recorded from rocks of Ordovician
age, but it was not until the Mesozoic that the group began to flourish.
They were especially abundant during the Cretaceous and have been
abundant and varied from that time until the present.
Echinoids are particularly numerous in many of the Lower Cretaceous
formations of Texas where they are commonly found in an excellent state
of preservation. Heart urchins and biscuit urchins may be found in large
numbers in many areas of the State, and especially in areas where there
are good exposures of fossiliferous Lower Cretaceous rocks.