The systematic position of the graptolites is uncertain. They are usually classed with the Hydroidea, as they show a general resemblance to some of the members of that group, namely the Sertularians. The rapid evolution of the grap tolites and the different aspects of the faunules of successive beds has permitted the recog nition of numerous graptolite zones in the Ordo vician. and Silurian rocks. In the Swedish Silu rian alone fifteen zones have been distinguished. Some of the graptolite zones exhibit an aston ishingly wide geographical distribution; a like succession of graptolite faunas has been found in Scandinavia, Great Britain, France, North Amer ica, and Australia. This extensive distribution, which gives the graptolites great importance for the correlation of geographically widely sep arated Paleozoic formations, is due to their hav ing been pelagic animals, which were either free floating or fastened to seaweeds floating in the open ocean. They are in sharp contrast to the great majority of the fossils, corals, mollusks, and crustaceans, which were littoral animals, living in the shallow waters near the shore. For this reason the graptolites are nearly always found separated from other fossils, in a facies or pecu liar rock phase of their own, the graptolite shales, mostly dark gray and black carbonaceous shales, upon the bedding planes of which they are crowded in immense numbers.
BIBLIOGRAPHY. Zittel and Eastman, Textbook Bibliography. Zittel and Eastman, Textbook of Paleontology, vol. i. (London and New York, 1900), where a short bibliographic list is given; Hall, "Graptolites of the Quebec Group," in Canadian Organic Remains, Decade IL, Geolog ical Survey of Canada (Montreal, 1865) ; Holm, various papers on the graptolites, in Geologiska Foreningens i Stockholm Forhandlingar (Stock holm, 1895-1900) ; Lapworth, "Notes on the Brit ish Graptolites," in Geological Magazine, vols. x. and xiii. (London, 1873-76) ; Wiman, "On Monograptus and Diplograptus," in Journal of Geology, vol. ii. (Chicago, 1893) ; Wiman, "Veber die Graptoliten," Bulletin of the Geological Insti tute of the University of Upsala, vol. ii. (Upsala, 1895) ; Ruedemann, "Synopsis of Recent Progress in the Study of Graptolites," in American Natu ralist, vol. xxxii. (Boston, 1898) ; id., "Growth and Development of Goniograptus," in Bulletin of the New York State Museum, No. 52 (Albany, 1902). See HYDROZOA; CCELENTERATA ; CAM BRIAN SYSTEM ; ORDOVICIAN SYSTEM ; SILURIAN SYSTEM.