Scyphocrinites.

Scyphocrinites (sky-foe-cry-NITE-ees) is a genus of crinoid that lived from the late Silurian to the early Devonian. Its fossils have been found in Asia, North America, Europe, and Africa.

Instead of being rooted to the ground like most crinoids, Scyphocrinites had a lobolith, which is a floating sphere that keeps an animal afloat in water. Unlike other crinoids, Scyphocrinites hung upside down at the surface.

This is a fossil of a lobolith from Scyphocrinites. The lobolith would have allowed Scyphocrinites to move with the current, which benthic crinoids could not do. Scyphocrinites was probably the only crinoid with a lobolith, but there may have been others.

This fossil shows the calyx of Scyphocrinites. In zoology, a calyx is a cup-shaped structure. In botany, it's the sepals that protect a flower. The arms coming off of the calyx would have allowed it to snare tiny plankton to eat. We know that because living crinoids use their arms for the same purpose. Fossils of Scyphocrinites are sometimes found in huge clusters, similar to how the Cretaceous crinoid Unitacrinus is found.

References:

http://www.fossilmuseum.net/Fossil_Galleries/Crinoids/Scyphocrinites/Scyphocrinites.htm

http://www.mineraltown.com/Reports/crinoid_fossils/crinoids.php?idioma=2

http://www.mprinstitute.org/vaclav/Scyphocrinites.htm

http://dictionary.reference.com/browse/calyx