Is ascribing a limb of one's wife to a limb of one's mother considered Zihar (an act of unlawful comparison)?

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If a man compares a limb of his wife to a limb of his mother, he is considered to have committed Zihar. This applies if he says, 'Your vulva (farjuki), or your back (dhahruki), or your head (ra'suki), or your skin (jilduki) is like the back of my mother, or her body, or her head, or her hand.' This view is held by Malik and is the explicit position of Al-Shafi'i. The justification is that he has compared her to a part of his mother, thus it is Zihar, similar to if he compared her entire self to his mother's back. This differs from comparing her to the limb of another wife, where Zihar would not occur. Furthermore, even if looking at that limb is permissible, deriving sensual pleasure from it is prohibited, and this is what the marriage contract permits.