Looks like a " Horseshoe Crab " fossil.
Yes, it's certainly not just a worn river rock. It's the fossilized exoskeleton of an anthropoid from a geological period of great antiquity (e.g. Cambrian, Devonian, Cretaceous, etc.).
It does show many similarities to the modern day Horseshoe Crab,so I assume it is an ancient ancestor, so it would seem logical to me to deduce that the occurrence of a fossil such as yours probably occurs throughout all geological periods, at least as far back as the Ordovician period ( roughly 450 million year ago according to present day scientific theory).
An interesting side note: modern day Horseshoe Crabs are captured off the east coast and transported to a lab in order to draw a portion of their blood, (before being released back into the ocean) which has very important medical field use! If anyone's interested interested in learning more, do a search on "horseshoe crab blood".
Mark, that's a Carboniferous Era horseshoe crab, but I don't think that's the rock posted by the original poster.
Yeah, horseshoe crabs are cool. The wife and I have returned a couple to Narragansett Bay when we found them in parking lots. I think it was kids having their idea of "fun" by laying them upside down away from the water. We've watched them in mating position too. The males are much smaller. The blood, BTW, is blue in color. I think their numbers are down here, no surprise, numbers down for lots of life forms as we seem to be in the Earth's 6th great extinction event...
Mating horseshoe crabs:
I don't think its a turtle shell, it looks more along the lines of a river/stream worn piece of rock that has strata lines/layer lines in it. Now Missouri has it's geologic/paleontological history that is unique. Think Elephant Rock and the pink rocks swimming hole, major major springs along the Eleven Point and Current/Jacks Fork that are very unique and large aquifers. I wouldn't bet on it being a cast of the interior of a shell either as it would have the ribbing on the top to show that. I think it's just a worn river or stream cobble. Could you tell us more about the location you found it in? River, Lake, on a hill, cliff, etc..