Improved Natural Attack( Monster Manual IV, p. 203)
The natural attacks of a creature with this feat are more dangerous than its size and type would otherwise dictate.
base attack bonus +4, Natural weapon,
The damage for one of the creature's natural attack forms increases by one step, as if the creature's size had increased by one category: 1d2, 1d3, 1d4, 1d6, 1d8, 2d6, 3d6, 4d6, 6d6, 8d6, 12d6. A weapon or attack that deals 1d10 points of damage increases as follows: 1d10, 2d8, 3d8, 4d8, 6d8, 8d8, 12d8.