The affixes biḍac and birīsac come in the sense of 'a hooked nose' after the word ni when the word so formed is a Name.,
Thus निबिडम् and निबिरीसम् ॥ How do you say निबिडाः केशाः or निबिडं वस्त्रम् ? It is by analogy.,
नेर्बिडज्बिरीसचौ
Adhyāya 5 · Pāda 2 · Rule 32