śamitā is formed irregularly by the elision of ṇi before an iṭ augmented affix when meaning a sacrificial act.,
Thus शृतं हविः शमितः ॥ It is formed by तृच् and is in the Vocative case. Why do we say 'when referring to a sacrificial act'? See शृतं हविः शमयितः ॥ See Satpatha Brahmana III. 8. 3, 4 and 5.,