During on visit to the Silica Shale dumps near Sylvania I was lucky enough to find some Arthrocantha fossils of my own.
This is a small Calyx with just the stumps of arms still attached.
This is a separate piece that shows part of the long arms that would be attached to the Calyx. Note the short spines at some of the arm junctions.
This species is well documented in the 1975 book by Robert V. Kesling & Ruth B. Chilman: Strata and Megafossils of the Middle Devonian Silica Formation: Museum of Paleontology, Papers on Paleontology, No. 8, pg. 166, 137, pl. 29, 37, 61, 62.