Arthrocantha crinoids were a common genera in the Silica formation and had a slew of protective spines to protect itself from predators. Still, some snails like Platyceras found their way onto the top of the animal. I'm not sure how the relationship worked, but the Platyceras would plant itself on top of the anal tube of the Crinoid and feed from it's waste. Considering that most specimens I've seen seem to cover the entire top of the Crinoid I'm not sure how the Crinoid survived.
These specimens were all purchased from a rock shop years ago.