This is a polished section of a fossil brittlestar (Ophiocrossota baconi) mortality layer from the Miocene aged Santa Margarita Formation of San Luis Obispo County, California. The layer is basically solid brittlestar fossils, and by cutting and polishing it the details can bee seen in cross-section. There are dozens of brittlestars present, most about 1" wide. The back of the slab is in a rough state, and brittlestars can be seen there too, though no much detail can be seen when they aren't in cross section.
One side has been cut flat so that it displays nicely standing up without a need for a display stand.