Arctinurus boltoni is one of the largest trilobites that can be found in the Rochester Shale of New York. It's parts are easily identified by their pustulose (small bumps on the surface of the carapace) nature.
I purchased this specimen of a pygidium from a rock shop in Basdell, NY (Past and Present). It is partially an impression with a little of the recrystallized carapace intact. The fossil has been painted with a preservative coating to prevent the fossil from falling apart.
The specimen came from the Rochester Shale which is dated to the Sheinwoodian/Homerian stage of the Wenlock epoch of the Silurian.
Here is a whole specimen on my friend Joe's website.
Mike at Lousiville Fossils has some pieces pictured on his blog from the Waldron Shale which is roughly equivalent to the Rochester Shale.