Tortoise Shell

Description
Tortoise shell grants a +6 enhancement bonus to the subject's existing natural armor bonus. This enhancement bonus increases by 1 for every three caster levels beyond 11th, to a maximum of +9 at 20th level. Tortoise shell slows a creature's movement to half its normal rate for the duration.

Though this spell's effects will stack with the natural armor of a subject, it will not stack with other enhancements to natural armor. E.g.: tortoise shell would stack with the bonuses provided by a bear's hide, but not with the bonuses provided by the barkskin spell.