The Hyolithida are molluscs, one of the two orders of hyolithid, the other being the Orthotheca. Most of our knowledge of the hyolithids comes from studies on the Hyolithida. Both orders had an operculum that was not hinged to the conch. However, the Hyolithida are distinct from the Orthotheca in lacking additional paired, curved, whiskerlike appendages. The Hyolithida were probably bottom feeders living in shallow water, and had tentacules.