ṣ is optionally substituted for the s when the preceding word is the name of a Lunar mansion.,
Thus रोहिणिषेणः or रोहिणिसेनः, भरणिषेणः or भरणिसेनः ॥ But not here शतभिषक् सेनः were the preceded letter is ग् ॥ These two sutras 8.3.99 and 8.3.100 are really Gana-sutras, being read in the सुषामादि Gana. The author of Kasika has raised them to the rank of full sutras.
All cerebral ष changes, when not referable to any specific rule, should be classified under this Sushamadi gana_.,