Improved Natural Armor
(Monster Manual v.3.5, page 304, DnD 3.5)
[General]
The creature's natural armor is thicker and harder than that of others of its kind.
Prerequisite
Natural armor, Con 13
Benefit
The creature's natural armor bonus increases by 1.
Special
A creature can gain this feat multiple times. Each time the creature takes the feat, its natural armor bonus increases by another point