The affix ṭhañ comes in the sense of 'come thence' after a word ending in short ṛ and denoting a person related through learning or blood.,
This debars वुञ् ॥ Thus होतुरागतम् = हौतृकम् 'derived from Hotri.' पौतृकम् ॥ So also of 'blood,' as:- भ्रातृकम्, स्वासृकम्, मातृकम् ॥
The त् in ऋत् is for the sake of facility of pronunciation, there being no such word ending in long ॠ ॥
When the words do not denote relationship through learning or blood, this affix is not employed. As, सावित्रम् ॥,