Extinct phylum of reef-forming organisms known only from the Cambrian. They were cup-like, usually 10–30 mm in diameter and up to 50 mm in height. In some respects they were similar to both sponges and corals, and may represent a true advance in evolutionary grade over the former. It is possible that they lived in a symbiotic relationship with some trilobites. The cause of their extinction is not known. There were two classes: Regulares and Irregulares.