In the chandas (Vedas), ṣ is substituted for the s when it stands in the beginning of a second word, preceded by a word ending in i etc.,
The words छन्दसि and एकेषां are both understood here. Thus द्विषन्धिः or द्विसन्धिः, त्रिषन्धिः or त्रिसन्धिः; मधुष्ठानम् or मधुस्थानम्, द्विसाहस्रं चिन्वीत or द्विषाहस्रं चिन्वीत ॥
The word पूर्वपद here means the first member of a compound word as well as the prior word other than that in a compound. Thus त्रिःषमृद्धत्वाय or त्रिःसमृद्धत्वाय ॥,