पृषोदरादीनि यथोपदिष्टम्

The elision, augment and mutation of letters to be seen in पृषोदर etc. though not found taught in the treatises of Grammar, are valid to that extent and in the mode, as taught by the usage of the sages.
Karika

वर्णागमो वर्णविपर्ययश्च द्वौ चापरौ वर्णविकारनाशौ ।
धातोस्तदर्थातिशयेन योगस्तदुच्यते पञ्चविधं निरुक्तम् ॥

The word यथोपदिष्टम् = शिष्टैरुच्चारितानि ॥ Thus पृषदुदरो यस्य = पृषोदरम्, पृषद् उद्वानं यस्य = पृषोद्वानम् ॥ Here there is elision of द् ॥ So also वारिवाहकः = बलाहकः, here वारि is replaced by व, and ल replaces व of वाहक; जीवनस्य मूतः = जीमूतः, here वन has been elided; शवानां शयनं = श्मशानम्; here श्म replaces शव; and शान for शयन ॥ ऊर्ध्वं खमस्य = उलूखलम्; here उल replaces ऊध, and खल replaces खम ॥ पिशिताशाः = पिशाचः ॥ ब्रुवन्तोस्यां सीदन्ति = बृसी ॥ Here सद् takes the affix उट् in the locative, and ब्रुव is replaced by बृ ॥ मह्यां रौति = मयूरः ॥ Here मही is replaced by मयू, and the final रु is elided before the affix अच् ॥ And so on with अश्वत्थ, कपित्थ &c.

Vart:- तीर becomes optionally तार after a word denoting direction, as दक्षिणतीरम् or दक्षिणतारम्, उत्तरतीरम्, or उत्तरतारम् ॥

Vart:- Before the Patronymic इञ् affix, वाग्वाद becomes वाड्वाल as, वाग्वादस्यापथ्यं = वाड्वालिः ॥

Vart:- The final of पथ् is changed to उ before दतृ, दश and धा (meaning 'location'): and the first letter of the second member is changed to ड or ढ ॥ Thus षड् दन्ता अस्य = षाडन्; so षोडश; षड्धा or षोढा कुरु ॥ The addition of उ is optional here. The putting of धासु in the plural number indicates that धा has here the meaning of "in many parts or ways" (नानाधिकरण). In fact it has the force of an affix here. The rule therefore does not apply to these:- षट् दधाति or धयति-षड्धा ॥

Vart:- The final of दुर् is changed to उ, before दाश, नाश, दभ and ध्ये and the first letters of these are changed to their corresponding cerebrals. Thus दुःखेन दाश्यते, नाश्यते, दभ्यते यः = दूडाशः, दूणाशः, दूडभः, In the last (दम्भ) there is elision of the nasal also. दुष्टं ध्यायति = दूढ्यः ॥ Here is added the affix क to the root ध्या (ध्यै) preceded by the upapada दुः, by (III.I.I36).

Vart:- उ is substituted for the final स्वर when the verb रुह follows in the Vedas: as एहित्वं जायेस्वो रोहावं (रोहाव is Imperative Ist. Person Dual).

Vart:- The final of पीवस् , पयस् &c. is elided in the Vedas. as पीवोप वसनानां, पयोपवसनानाम् ॥