Platyceras was a snail with a broad, rounded shell. It lived in symbiosis with various stalked echinoderms , often attaching itself to the echinoderm’s anus and feeding on the faeces.
The rim of the shell was slightly irregular, enabling it to fit the crinoid’s shape closely.
Whether the host obtained any benefits from the presence of Platyceras remains unclear. Fossil Platyceras specimens are sometimes found still in place on the fossil host animals.