Fig. 11. Morphology and principal parts of corals. (a) [Solitary] or “horn” [coral]. (b) [Colonial] or compound coral.
a [Columella] [Septum] [Corallite] b [Calyx] Septum [Corallum]
The class Anthozoa has been divided into several subclasses, but only one, the Zoantharia, is of paleontological importance.
Subclass Zoantharia.—
Most corals and all sea anemones belong to this subclass. Zoantharians are either [colonial] or [solitary] and, because most of them possess a hard preservable [exoskeleton], they are the most important group of anthozoans geologically. The various orders of the subclass Zoantharia are discussed below.
Order Rugosa.—
These are corals in which the septa are arranged in cycles of four. Both [solitary] and [colonial] forms occur, and they are found only in rocks of [Paleozoic] age. Rugose corals are abundant in many of the Paleozoic formations of Texas, and two of the more typical forms (Lophophyllidium and Caninia) are illustrated in [Plate 15]. Members of this order have been placed in the subclass Tetracoralla of older classifications.
Order Scleractinia.—
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.