Need help to identify the following fossil. It was found in Milos, preserved in volcanoclastic sediment and diatomite. Data from a study in the area suggest late pliocene to early pleistocene. Looks like an echinoderma, but unlike any I've ever seen, since it's test appears to have layers and no apparent mouth (unless it's on the bottom part). I have even considered it being a cystoid or some coral, but I haven't had any luck researching my theories. Units on the pic with the ruler are cm and size is about 4.5cm. Any suggestion would be much appreceated!