I have no information about the origin of this fossil, except to say that it came from a large bag of pea pebbles purchased at a big box store in the St. Charles, Missouri area. The raised ridge in the center made me wonder if it's a trilobite. There's also an interesting "shift" about midway down the ridge that creates an offset of sorts. Maybe that's the result of settling over time? It measures about 0.75 inch long, as shown in one of the photos.