Strobilocystites calvini
Cystoid

Mid-Late Devonian
Cedar Valley Formation – Rapid Member
Palo, Iowa, USA

43mm cystoid on 92mm matrix.

Strobilocystites is a genus of cystoid known from the Devonian of North America and was one of the late-surviving members of the group.

Cystoids belong to an extinct order of echinoderms from the Ordovician to Devonian Periods.  Similar to crinoids, they had somewhat rounded, plated, bodies attached to a stalk that was anchored to the sea bottom. 

This is a very nice three-dimensional example of S. calvini in matrix.