Pleurocystites is a somewhat common cystoid found in the middle to upper Ordovician. I've managed to find one in the Verulam formation of southern Ontario. The specimen displayed below is a Pleurocystites squamosus and was cleaned off by my friend Kevin. It is associated with a Rafinesquina alternata (white shell), a couple of what I think are Plaesiomys brachiopods along with other brachiopod and bryozoan debris.
A closer view of the calyx. You can see just the stumps of the two arms that once sprouted from the top of the calyx. Unfortunately they must have weathered away or were on another piece of rock that this one broke off of.
One of the interesting features are the "vent" like openings, called dichopores, that are found in three places around the body. This is a closer view of one. Based on what information I have been able to find so far, dichopores were respiratory structures.
The specimen above was found in the Verulam formation which is upper Ordovician in age (Katian stage, equivalent to the lower Caradocian stage in Europe and the Mohawkian stage in the US). Thanks to my friend Kevin for the field ID and for cleaning the fossil off in his prep lab.