All I can tell you is if someone brought that into our archeologists for ID, they would likely just hear that it is a tool, possibly a knife of unknown age. I'd say that tools like that were produced from Paleo to Mississippian times and could serve many functions from scraping to wood working due to the bevel. Since so many cultures and cultural periods could have produced it and it was found outside of archeological context, pinning down a name would be difficult. Your only real hope in my opinion is to find someone who is very familiar with the archeology of your area and could inturn make an educated guess based on that familiarity.
- That was a long-winded way of saying "I don't know!" LOL
Maybe someone with one of those Field Guide to Stone Artifacts of Texas will chime in.