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Crinoid

species, crinoids, arms, period, fossil and limestones

CRINOID, kri'noid, or a stalked echinoderm usually fixed to the sea bot tom by a jointed stem so as to present a flower like form. The body is more or less cup shaped, with 5 to 40 jointed flexible arms subdivided into branches, and bearing pinnules, all made up of minute bony plates, which in some of the larger fossil species numbered over 150,000. The arms may be absent in the blas toids (Pentremites) and certain cystideans, but the pinnules remain. There are not now many existing species, the greater number (nearly 1,000) having become extinct A typical crinoid is Pentacrinus, which lives attached to rocks in the West Indies at all depths from 20 to 3,000 fathoms; it is about a foot high, the arms much subdivided, the joints of the stem five-sided. In one fossil species the stalk was more than 50 feet long. In geologic times crinoids often grew in dense forest-like masses. A curious little living crinoid is a slender simple form about two inches high, which lives at the depth of from 100 to 1,000 fathoms on the coast of Norway and in the Straits of Florida in the cold water under the tepid Gulf Stream. It is a sur vivor of a genus of the Cretaceous Period. A north Atlantic shoal-water form is the Antedon (Comatula), which in its early youth is fixed to the bottom by a stalk, but which becomes free when mature; it also inhabits the Mediterranean Sea. The existing crinoids, more than 200 spe cies, are merely the remnants from a much larger assemblage of fossil forms, which begin to appear in the rocks of the Cambrian, culmi nate in the early Palmozoic and decline toward the end of that period. At first small and deli cate, they became larger and coarser in the later rocks. They flourished in greatest numbers about palaeozoic coral reefs in shallower water than at present. The most famous American fossil crinoid beds are those of the Subcarbon iferous limestones of Burlington, Iowa and Crawfordsville, Ind. Thick beds of crinoi

dal limestones were deposited in various parts of the world at various periods and under favorable conditions from the Ordovician to the Jurassic Period, those of the Carboniferous and of the upper Muschelkalk, the lower beds form ing the so-called Trochitenkalk, being especially characteristic, and consisting almost wholly of stems of Encrinus liliformis, the "stone lily.' Crinoids are divided into three classes. The oldest, most generalized and primitive appears to be the class Cystoidea. These were more or less spherical in form, either with imper fectly developed arms, or without, and stalked or not. About 250 species are known. They date from the Cambrian Period, culminated in the Ordovician and Silurian Periods, then sud denly diminished in numbers, finally disappear ing before the close of the Permian. The sec ond class is the BlastOidea, or bud-shaped crinoids, represented by Pentrenates, which were short-stemmed or entirely stemless. The arms are short, recumbent and appear as if soldered to the calyx or body. These have not yet been detected in strata lower than the Silurian and the type became most numerous in the Subcarboniferous limestones of the Unite' States. Upward of 120 species have been recog nized. The third class is Crinoidea proper. The three classes are arranged under the sub-branch, Peimatozoa. New species are constantly being found, notably in the waters of the Philippines, Africa and the Hawaiian Islands, and a few along the Atlantic Coast of the United States. Consult Clark, A. H., 'A Monograph of Exist ing Crinoids) (United States National Museum Bulletin 82, Washington 1915) ; Proceedings of the United States National Museum, Vols. 34 onward: Zittel, K. A. von (Eastman's trans.), 'Text-book of Pala-ontology' (London 1900 02).