horn coral

A curved cone, wide at one end and pointed at the other, with fine ribs radiating inside the wide end like the spokes of a wheel.

A horn coral is a rugose coral that lived alone instead of in a colony, which is why it grew a horn instead of a honeycomb. Plenty of rugose corals were colonial, the petoskey among them, so rugose does not mean solitary.

Devonian corals laid down a fine growth line every day. In 1963 John Wells counted about four hundred of them between one year band and the next, which says the Devonian year held four hundred shorter days. The earth was spinning faster.