There is the substitution of śīrṣa for śīras when a taddhita-affix beginning with a vowel follows.,
Thus हस्तिशिरसोऽपत्यं = हास्तिशीर्षिः formed by adding the Patronymic affix इञ् 4.1.96. So also स्थूलशिरस इदम् = स्थौलशीर्षम् ॥ Had the word been शीर्षन् (instead of शीर्ष as taught herein) then it would have retained its final न् before these affixes and would not have given the proper forms 6.4.168. Again in forming the feminine of हास्तिशीर्षिः by adding ष्यङ् (IV. I.78), arises this difficulty :- ष्यङ् (य) is a Taddhita affix beginning with य, when this is applied to हास्तिशीर्षिः, we must apply the last rule and change the शीर्ष into शीर्षन् (for शीर्षन् being the substitute of शिरस् is prima facie a substitute of शीर्ष also for the purposes of that rule (VI. I. 61)). The form which we get will be this, हास्तिशीर्षिः + ष्यङ् = हास्तिशीर्षण् + य (VI. I. 61) = हास्तिशीर्षण्या 6.4.168. But this is a wrong form, the desired form is हास्तिशीर्ष्या ॥ How do we explain this? Thus हास्तिशीर्षि + ष्यङ् = हास्तिशीर्ष् + य (the इ is elided by 6.4.148 and in the room of इ we substitute a zero or लोपादेश)॥ Now this adesa becomes sthanivat to इ, thus the affix य not being directly applied to शीर्ष, because this zero intervenes, शीर्ष is not replaced by शीर्षन् as required by 6.1.61, for it is not followed by an affix beginning with य but by a zero sthanivat to इञ् affix.
This sutra is not of Panini, but is really a Vartika raised to the rank of a sutra by later authors.,