The scleractinians are solitary or colonial corals in which the septa grow in multiples of six, and they are the most important and abundant of the modern corals. These corals were the dominant reef builders of Mesozoic and Cenozoic seas, and their remains are common in many of the marine formations of the State. Plate 16 illustrates some typical scleractinian corals from the rocks of Texas. This order has also been referred to as subclass Hexacoralla, and its members have been called hexacorals. Order Tabulata.— These are corals that are now extinct but are known from fossils in both Paleozoic and Mesozoic rocks. Tabulate corals are characterized by horizontal partitions called _tabulae_, and septa are absent or poorly developed. The tabulates were the most abundant reef-building corals during Paleozoic time and are well known as fossils. Because of certain similarities with other anthozoans, some paleontologists have treated the Tabulata as a distinct subclass rather than as an order of the Zoantharia. Tabulate corals are not uncommon in many of the Paleozoic rocks of Texas, and two of these (_Cladochonus_ and _Striatopora_) are illustrated in Plate 15. Phylum Bryozoa [Illustration: Fig. 12. Two types of bryozoans or “moss animals.” (a) Section of the lacy type bryozoan. (b) The spiral axis of _Archimedes_ (Mississippian).] Bryozoans are colonial animals that are often referred to as “sea mats.” They have been called this because they are commonly found matted on shells, rocks, fossils, and other objects. The living animal is quite small, has a tentacle-bearing ridge surrounding the mouth, and secretes a tiny cup-like exoskeleton composed of calcareous or chitinous material. These little chambers, known as _zooecia_ (or _autopores_), are seen as small pits on the surface of the bryozoan colony (_Rhombopora_, Pl. 17). The zooecia grow together to form the bryozoan colony, and some fossil colonies grow to be as much as 2 feet across. Such colonies may be spiral (fig. 12b), branching, or lace-like (fig. 12a), and the latter two types are very common in many of the fossiliferous strata of Texas. Undoubted bryozoan fossils have been recorded in rocks of Lower Ordovician age, but questionable Cambrian forms have also been reported. Bryozoans are abundant in the seas of today, but only a few forms inhabit fresh waters. [Illustration: Plate 14] [Illustration: SPONGE SPICULES (GREATLY ENLARGED)] [Illustration: PALEOZOIC SPONGES] MEANDROSTIA × 1 HELIOSPONGIA × 1 ASTRAEOSPONGIUM × ½ ASTYLOSPONGIA × ½ RECEPTACULITES × ½ GIRTYOCOELIA × 2 [Illustration: Plate 15 PENNSYLVANIAN CORALS]