Hyolitha

Hyolitha are enigmatic animals with small conical shells. They are known
only from the Cambrian era. The conical shells have 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.

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