Hyolitha are known only from the
Cambrian. Conical shells with a cover (
operculum[?]) and two curved supports known as Helens. The shells are calcerous and are usually triangular or elliptical in cross section. Some species have rings or striations. Most are one to four centimeters in length. Some authors treat them as molluscs, but it is generally agreed that there is no very strong argument to do so.
Despite the fact that Hyolithid shells are quite common as fossils, next to nothing is known about their ancestry and internal structures. They were obviously bottom dwellers. There is some evidence that they were carnivores.