tentaculites

from The Century Dictionary.

• noun A genus of small elongated conic shells of circular section, the outer surfaces of which are marked by strong regular transverse ridges and by fine longitudinal striæ on the spaces between the rides. They are confined to the Paleozoic and are extremely abundant in many Silurian and Devonian formations. They have usually been regarded as the shells of pelagic Pteropoda, but many authorities consider them as annelidan tubes.
• noun The typical genus of Tentaculitidæ, having such species as T. irregularis.

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

• noun Plural form of tentaculite.