This is yet another invertebrate fossil that has perplexed me. In the field I thought it was an ostracod, but under closer inspection it has shown itself to be something different. I don't think that it is a brachiopod piece this time, but bryozoan would possibly fit. Another possibility I thought of was the end of an extremely small trilobite glabella, with a dent running through the center. Yet another possibility is nothing, but it is pretty symmetrical, so I think it is a fossil.
I presume it is from the Hersey Formation. I found it on the seashore, and it seemed to be brought by the current. There are ostracods in it that match with what I know of this formation, but I am uncertain. All the formations in the area, though, are Silurian.
It is approximately 900 microns long.
Here are pictures of it: