ṣ is optionally the substitute of the s of skand after the upasarga (Preposition) pari everywhere.,
Thus परिष्कन्ता or परिस्कन्ता, परिष्कन्तुम् or परिस्कन्तुम्, परिष्कन्तव्यम् or परिस्कन्तव्यम् ॥
The word परि could well have been included in the last sutra as विपरिभ्यां स्कन्देर् &c. The very fact that it has not been so included, indicates that the prohibition of अनिष्ठायाम् does not apply to it. Thus परिष्कण्णः or परिस्कन्नः ॥,