UCMP MYSTERY FOSSIL #40 is . . .
the colonial coral Syringopora

Syringopora is a tabulate coral, so named because of the horizontal internal partitions (tabulae) found within colonies. Most tabulates were colonial, with some forming substantial reefs. The tabulate corals were victims of the largest mass extinction ever, 245 million years ago at the close of the Permian. Find out more about corals and other anthozoans here.