Matsya 124.54 states its three asterisms: Aśvinī, Kṛttikā, and yāmyā (= Bharaṇī, whose deity is Yama). Wilson's note 23 to VP 2.8 gives the same: 'The stars of the Nágavíthi are those of Aries and Taurus.' In Viṣṇu P. 2.8.90 it is a latitude marker: the Devayāna lies north of Nāgavīthī and south of the Saptarṣis. Homonym caution: Nāgavīthī is also a woman in the genealogies (daughter of Yāmī) at VP 1.15.107 — the star-road personified.
नागवीथी Nāgavīthī
the Serpent-road, northernmost of the nine star-roads · — · Aries and Taurus (Aśvinī, Bharaṇī, Kṛttikā)
Attestations
Transcribed from the Devanagari OCR of the printed edition at shloka_source_url (Achyuta Nand Jha ed., vol. 1) and checked word-for-word against two further independent scans of two different printed editions of the same commentary — Krishna Chandra Dvivedi's (https://archive.org/download/brihatsamhitawithbhattotpalavivrittipartidr.krishnachandradvivedi/Brihat%20Samhita%20%20with%20Bhattotpala%20Vivritti%20Part%20I%20-%20Dr.%20Krishna%20Chandra%20Dvivedi_djvu.txt) and A.V. Tripathi's (https://archive.org/download/JiUm_brihat-samhita-part-1-of-varaha-mihira-by-avadh-bihari-tripathi-saraswati-bhavan-granthamala/Brihat%20Samhita%20Part%201%20of%20Varaha%20Mihira%20by%20Avadh%20Bihari%20Tripathi%20-%20Saraswati%20Bhavan%20Granthamala_djvu.txt). Only glyph-level OCR artifacts that all three scans show to be artifacts (e.g. OCR रद्वव for रद्गव, श्व for श्च) have been normalised; every substantive reading is as scanned, and the two places where the scans genuinely differ are flagged in identification_notes.
FIVE INDEPENDENT SCHEMES, four of them pre-Varāhamihira, are set side by side by Utpala in ch. 9 — this is the single densest surviving witness to the vīthī system, and the database's vīthī entries hitherto rest on the Purāṇas alone. (a) DEVALA and others: nine triads straight from Aśvinī, Nāgā to Dahanā. (b) KĀŚYAPA: the same triads, given asterism by asterism. (c) VARĀHAMIHIRA'S OWN (BS 9.2–3), which starts the Nāgā with Svātī instead of Aśvinī and gives the Go-vīthī four asterisms and the Dahanā two. (d) PARĀŚARA, differing again (Airāvatī four, Vṛṣabhā two, Mṛga-vīthī two, Ājī = Anurādhā/Viśākhā/Jyeṣṭhā, Vaiśvānarī = Mūla + the two Āṣāḍhās). (e) GARGA, close to Varāhamihira. Plus (f) the SAMĀSA-SAṂHITĀ, Varāhamihira's own lost epitome, agreeing with (c). Utpala explicitly notes the clash — 'atra varāhamihireṇa saha bhedaḥ', 'here there is a difference with Varāhamihira' — and Varāhamihira himself refuses to adjudicate at 9.7. For the NĀGA-VĪTHĪ specifically the split is sharp: Devala and Kāśyapa make it Aśvinī, Bharaṇī, Kṛttikā; Varāhamihira, Garga, Parāśara and the Samāsa-saṃhitā all replace Aśvinī with the far-off SVĀTĪ (Parāśara phrasing it by deity: 'āgneya-yāmya-vāyavyāni'). Utpala also supplies the variant name VYĀḌA ('serpent') for the same road, in his chapter opening.
See also
Airāvata / Jaradgava / Vaiśvānara (the three mārgas), Pitṛyāṇa / Devayāna