195. The immediate combination

a. The immediate combination of a n with a preceding guttural or labial seems in some cases to hinder the conversion to ṇ: thus, vṛtraghnā́ etc., kṣubhnāti, tṛpnoti (but in Veda tṛpṇu), kṣepnú, suṣumná.

b. The RV. has the exceptions úṣṭrānām and rāṣṭrā́nām.