Bharata Natyam
Elizabeth Kelly
Śankha (conch): the thumbs of Sikhara hands are joined, and the forefinger extended.
conch
Śaṅkha. When thumb of a Sikhara hand meets the other thumb and is clung round by the fore-finger [close to the latter thumb] the hand is called Śaṅkha.
It is used to denote conch-shell and such other things.
When the thumbs of two Sikhara hastas are joined and similarly the forefingers etc. are also joined it becomes Saṁkha hasta (Saṁkha=conch).
This hasta is used to denote the conch (i.e. blowing the conch) etc. as mentioned by Bharata and others.