Surya Siddhanta
ध्रुवाणि शकुनिर् नागम् तृतीयम् तु चतुष्पदम् । किम्स्तुघ्नम् तु चतुर्दश्याः कृष्णायाश् चापरार्धतः॥
The four invariable Karaṇas called S'akuni, Nága, Chatushpada and Kinstughna (always appropriate to themselves successively the halves of the tithis,) from the latter half of the fourteenth tithi of the dark half (of a lunar month to the first half of the first tithi of the light half of the next lunar month inclusive).
english translation
dhruvANi zakunir nAgam tRtIyam tu catuSpadam । kimstughnam tu caturdazyAH kRSNAyAz cAparArdhataH॥
hk transliteration by Sanscript