Wikipedia
Platyceratidae
Platyceratidae is an extinct family of Paleozoic sea snails, marine gastropod mollusks. This family may belong in the Patellogastropoda or the Neritimorpha. The platyceratid gastropods are known for the complex symbiotic relationships they had with crinoids.
This is the only family in the superfamily Platyceratoidea.