Fig. 42.—Living Anthozoan Coral (Astræa).
In their general plan, the oldest Corals were precisely of this character, but they presented some differences in detail, which have caused them to be divided into two groups, which are eminently characteristic of the Palæozoic age—the tabulate or floored corals, and the rugose or wrinkled corals. In the former ([Fig. 43]) the cells are usually small and thin-walled, often hexagonal, like a honeycomb, and are floored across at intervals with tabulæ or horizontal plates. A few modern corals present a similar arrangement,[14] but this kind of structure was far more prevalent in the Palæozoic. In the second type the animals are usually larger and often solitary, the cell has strongly marked radiating plates, while the horizontal floors are absent or subordinate, and there is usually a thick external rind or outer coat ([Figs. 44, 45]). In general plan, these rugose corals closely resemble those of our modern reefs; but they differ in their details of structure, and only a very few modern forms from the deep sea are regarded as actual modern representatives.[15] One curious point of difference is that their radiating laminæ begin with four, and increase by multiples of that number, while in modern corals the numbers are six and multiples of six; a change of mathematical relation not easily accounted for, and which assimilates them to Hydroids on the one hand, and to a higher group, the Alcyonids, on the other, both of which prefer four and eight to six, or have had these numbers chosen for them. In the Mesozoic period the tabulate and rugose corals were replaced by others, the porous and solid corals of the modern seas; but, in so far as we know, the animals producing these, though differing in some details, were neither more nor less elevated than their predecessors, and they took up precisely the same rôle as reef-builders in the sea, though with probably more tendency to the accumulation of great masses of coral limestone in particular spots.
Fig. 43.—Tabulate Corals.
a, Halisites, and b, Favosites. Upper Silurian.
Fig. 44.—Rugose Coral (Heliophyllum Halli). Devonian.