I found this near my home in the Texas Hill Country. The area is within the Balcones Escarpment. Oddly enough, it was just laying on top of the ground. All I had to do was pick it up.
I need to get a Texas Fossil Handbook, but all I have at the moment is the National Audubon Society: Field Guide to Fossils. I found two fossil times that may be a match, though they are not exactly the same--Metoicoceras and Texanite. I believe my fossil is an interior mold, but I'm not sure.
Thanks for helping me identify it.