FOSSIL FORMS, Among the fossil corals only those belonging to the Anthozoa, are of importance, those of the Hydromeduste either not appearing until the Mesozoic, and then but sparingly (for fossil Hydroeorallime and Tubularire, see CCELES TERATA ) or not being clearly recognized as to their systematic position and only provisionally referred to the Ifydrozoa. ( See STROMATOPORA and GRAPTOLITE.) The Anthozoa appear in the Cam brian rocks with the Archreocyathime, forms of peculiar structure and uncertain relationships. In the Upper Silurian period they have become the most important fossils, as well in number of in dividuals as in diversity of structure and impor tance as rock-building organisms. They continue thus throughout geologic times, and are hence of great importance to the paleontologist and geolo gist. As they are also objects of great beauty, they are much sought by collectors and dealers. Especially rich and famous are the fossil coral faunas of the Silurian of North America and Got land; the Devonian of the Helderberg.s of New York, the Falls of the Ohio, and the Rhenish provinces; the Jurassic formations of Middle Europe and the Tertiary beds of northern Italy.
The Anthozoa are divided by Haeckel into the sub-classes Tetracoralla. Ilexacoralla, and Octo coralla, according to the number of their septa. which were considered to be multiples of these figures. The Tetracoralla comprise, together with the Tabulata, the Paleozoic corals; the Hexa coralla and Octocoralla. the Mesozoic and later forms. Recent investigations on the embryology of the Hexacoralla and thecal structure of the Tetracoralla (Dr. M. Ogilvie) seem to demon
strate that these divisions are artificial, that the tetrameral system is only an ancestral feature strongly marked in certain of the old families ( Cya thophyll id re, Zaphrent idre, Cya tha xon Ida.) , while hexameral symmetry is but one of many forms of radial symmetry. The Anthozoa are therefore at present divided into but two sub classes, according to Ray Lankester: the Aleyo naria or Oetoeoralla, and the Zoantharia. None of the fossil alcyonarians, which with doubtful forms do not appear till the Mesozoic. are of importance; the Paleozoic Heliolitidre, which are referred with doubt to this sub-elass, are quite abundant in the Silurian and Devonian rocks.
The sub-class Tabulatze, of Milne-Edwards and Haime, comprising common and important Paleo zoic genera, such as Favosites, Aulopora, Syringo porn, Halysitcs, and Ch:etetes, which was long considered as a distinct characterized by the slight development of the septa and the presence of numerous tabulre, has now been broken up: it having been recognized that some families belong to the Octocoralla, others (Favositidm. Syringoporidte, Halysitidre) exhibit close relationships to the Hexacoralla, and others (Cketetidre, Montienliporidre) are even placed by some authors, as among the Polyzoa. All these very primitive groups of tabulate corals played important roles as Pale ozoic reef-builders.