[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. Subclass Holothuroidea.— Members of this class, commonly called _sea cucumbers_, have a rather elongate, sac-like, cucumber-shaped body and bear little resemblance to other members of the phylum Echinodermata. The sea cucumbers do not have a well-defined skeleton; rather the body is supported by many small, disconnected, calcareous plates or rods called _ossicles_ or _sclerites_ (Pl. 35). These minute structures are embedded in the leathery skin which covers the body of the sea cucumber and may be preserved as fossils. Such remains are locally abundant in certain formations in Texas, but because of their small size, scattered occurrence, and problems in classification, this group is of little use to most paleontologists. Holothuroid body impressions have been reported from the Middle Cambrian, and sclerites from rocks as old as Mississippian. Phylum Chordata The chordates are the most advanced of all animals and are characterized by the presence of a well-developed nervous system and a body supported by a bony or cartilaginous _notochord_ and/or _spinal column_. In the higher chordates (the vertebrates) the notochord is normally replaced by bone, but in the lower chordates (for example, the graptolites) it remains in a cartilaginous condition. The phylum Chordata contains only two subphyla of paleontological significance. These are the subphylum Hemichordata, composed of primitive chordates (including the graptolites which are important fossils), and the Vertebrata, which includes all animals with backbones. Subphylum Hemichordata