The feminine affix ṅīṣ is not used after 1. nakha 2. mukha when (the word at the end of which they stand) is a Name.,
Thus शूर्प + नखा, = शूर्पणखा, 'Surpanakha the sister of Ravana'. The न् is changed into ण् by 8.4.3 (पूर्वपदात् संज्ञायामगः) ॥ So also वज्रणखा, गौरमुखा, कालमुखा &c. Why do we say 'when denoting a Name'? Witness ताम्रमुखी कन्या 'a copper-faced damsel', or चन्द्रमुखी 'moon-faced'.,