Your mollusk fossil is a gastropod - a snail - from the Lower Cretaceous of Texas. More accurately, the fossil is a steinkern - an internal cast - of the snail.
After burial, the snail shell dissolved away, but not before it filled with fine sediment from the surrounding limey sediment. That limey sediment hardened into the cast you have today. It appears to be a bit stream-abraded by the Brazos, but is a fine souvenir of your outing.