FOSSIL FORMS. The Ccelenterata comprise some of the oldest fossils found in the Lower Cambrian rocks (Arclneocyathus and Ethmophyl lum) ; they increase rapidly in variety, and tained already. early in the Silurian, great portance:is ock-builders in the reef-build ing cora Is (see CORAL. ISLAND; LIMESTONE; DOLOMITE), which persisted through all formations to the present time, their remains often forming the greater part of whole mountain ranges, as in the Southern Alps. Medusoid impressions, repre senting not only the fiydromednsir. but also the Scyphomedusie, occur as early as the Cambrian in Bohemia, Sweden, and North America, and are known from later rocks. (Sec MhotisA; JELLY Fisn.) Those suborders of the Hydr0medus4e. whose species possess either ehitinous-caleareous or calcareous skeletons—viz, the Hydroco•allime and Tubularke—are also found in the fossil con dition, though only sparingly and not until the Mesozoic and Tertiary formations. The most important representatives of the first suborder are the calcareous skeletons of Millepora, appear ing in the Eocene, and of the second sub-order, the masses of calcareous concentric lainelke with supporting pillars, which have been described as Ellipsactinia from the Alpine Jurassic, and as Parkeria. and Porosphaera from the Cretace
ous. Similar calcareous concentrically lamellose forms. but without larger apertures on the sur face, the Stromatoporoidea (see STRoaATOpoRA), are extremely common in the Silurian and De vonian systems, where they were reef-builders and important rock-making organisms, being associated with the corals. Their real systematic position being unknown, they are provisionally allied with the Hydromedus.c.
A like relation to the Hydromedusic is held by the graptolites (see GRAPTOLITA), which, form ing chitinons hydroid-like colonies, swarmed, either as holoplanktonic or pseudoplanktonie or ganisms, in the Silurian seas. They differ in some important features, as the possession of a sicula and virgula, from the hydroids. On ac count of the world-wide distribution of their spe cies and their short range, they are most impor tant ho•izon-markers or index fossils.
For bibliography of fossil forms, see the ar ticles on CORAL; GRAPTOLITA; STROMA TOPORA; JELLY-Fishi ; etc.
See also ACALEPILE; ALCYONARIA; SEA-ANEM ONES; ANTII0Z0A; HYDROIDS; POLYPS; PORTE CUESE MAN-OP-WAR; SEA-FANS; SEA-PEN; SI