graptolite

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of numerous extinct colonial marine animals chiefly of the orders Dendroidea and Graptoloidea of the late Cambrian to the early Mississippian Periods, whose fossil remains are often used to date the rocks of the Silurian and Ordovician ages.

from The Century Dictionary.

• noun One of the Graptolithidœ, Graptolithina, or Rhabdophora; a specimen or a species of Paleozoic cœlenterate organisms, commonly supposed to be hydrozoans, resembling the living sertularians in having a horny polypary, and in having the separate zoöids protected by little horny cups, all springing from a common cœnosarc, but differing in that they were not fixed to any solid object, but were permanently free.
• Same as graptolitic: as, a graptolite schist.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Paleon.) One of numerous species of slender and delicate fossils, of the genus Graptolites and allied genera, found in the Silurian rocks. They belong to an extinct group (Graptolithina) supposed to be hydroids.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of a group of extinct aquatic colonial invertebrates, of the class Graptolithina, from the Cambrian and Carboniferous periods