The second geological period of the Palaeozoic era, following the Cambrian and preceding the Silurian periods. It began about 485 million years ago and lasted for about 41 million years. The period was named by the British geologist Charles Lapworth (1842–1920) in 1879. Graptolites, in deep-water deposits, are the dominant fossils. Other fossils include trilobites, brachiopods, ectoprocts, gastropods, bivalves, echinoids, crinoids, nautiloid cephalopods, and the first corals.

http://www.ucmp.berkeley.edu/ordovician/ordovician.php A brief survey of the Ordovician period on the website of the University of California Museum of Paleontology