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
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. 12]b), branching, or lace-like ([fig. 12]a), 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.
Plate 14
SPONGE SPICULES
(GREATLY ENLARGED)
