RANGE. The Cystoidea is the oldest known class of echinoderms; their isolated plates. rare ly united to give a clue to the form of the ani mal, known under the names of Eoeystites. Pro toeystites, etc., from the Cambrian rocks, are the earliest representatives. The class enjoyed two periods of expansion. First. in the early Ordovician time they flourished in hosts in some regions, their remains forming the larger part of certain limestones, such as the lower Chazy limestones of Lake Champlain. Other lime stones. of Beekmantown and Trenton age. in the Saint Lawrence and Champlain valleys, and beds of equivalent age in the Baltic provinces of Eu rope. contain abundant cystoid remains. The second expansion of the class occurred during the Silurian time, when these creatures lived in abundance in some portions of the seas of north ern and middle Europe and eastern North America. In all about 250 species are known.
and of this number only about 15 have beer, found in rocks above the Silurian system. The
group entirely disappeared with the close of Paleozoic time.
Consult: Forbes, "On the C'ystidea of the Si lurian Bucks. of the British Islands," Memoirs of the Geological Surrey of Cur zt Britain, vol. ii.. part 2 (London. ISIS) ; Billings, "On the Cys of the Lower Silurian of Canada," Figures and Descriptions of Canadian Organic ttemains, Decade III. (\lontreal, 185S); Ilall, "Descriptions of Some New Fossils from the Niagara Group." Twentieth Annual Report of the etc York Stale Cabinet of Natural History (Albany, 1567) ; Barrande, "Cystidi7-es," SystCnze ASilurien du Oentre de If, B011enle, Vol. Vii.
(Prague and Paris, 18'37) : Bather. "The Cysti dea," iu Lankester's Treatise on Zoology, part iii., chap. ix. (London, 1900) ; von Zittel and Eastman, Textbook of Paleontology, vol. i. (Lon don and New York, 1900). See CR1NOIDEA ; ECIIINODERMATA ; and articles on the other classes of echinoderms.