The affix ṭac comes after an avyayībhāva samāsa ending in giri according to the opinion of the Grammarian senaka,
Thus अन्तर्गिरम् or अन्तर्गिरि, उपगिरम् or उपगिरि ॥ The name of Senaka is mentioned for the sake of respect (pujartha), for the anuvritti of the word अन्यतरस्याम् \optionally\ was understood here already.,