For the visarga of adhas or śiras when not preceded by another word and followed by the word pada in composition with it, there is substituted s,
Thus अधस्पदम्, शिरस्पदम् ॥ अधस्पदी, शिरस्पदी ॥
The word समासे is understood in this, therefore not here अधः पदम् ॥
The word अनुत्तरपदस्थस्य is also understood here. Therefore not in the following परमशिरः पदम् ॥
The word अधस्पदम् is a compound formed under Mayuravyansakadi class.,