The affix ṣa comes after a bahuvrīhi samāsa ending in mūrdhn and preceded by 1. dvi and 2. tri,
Thus द्विमूर्धः, त्रिमूर्धः ॥ The feminine is formed by ङीष् and the accent is regulated by 6.2.197.
Why do we say \preceded by dvi and tri?\ Observe उच्चैर्मूर्धा ॥,