Natyashastra
विनिष्क्रामो विसर्गस्तु प्रवेशो विनिगूहनम् । सन्दष्टकं द्विजैअर्दष्टं समुद्गः सहजोन्नति ॥ १४४॥
Visarga (spreading out)—to spread out (lips). Vinigūhana (concealing)—drawing in (of lips). Sandaṣṭaka (biting)—(lips) bitten by teeth. Samudgaka (joining)—moving the lips together.
english translation
viniSkrAmo visargastu pravezo vinigUhanam । sandaSTakaM dvijaiardaSTaM samudgaH sahajonnati ॥ 144॥
hk transliteration by Sanscript