2.4.2 द्वन्द्वश्च प्राणितूर्यसेनाङ्गानाम्

Word by word (padaccheda)

Substitute / term prathamā · ekavacanaIndeclinable च avyayaTarget प्राणि- तूर्य- सेना- ṣaṣṭhī · bahuvacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

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Full reading

च प्राणि-तूर्य-सेना-

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

An affix which begins with the augment इट् -- आर्धधातुकस्येड् वलादेः 7.2.35, is ङित् after the root विज् 'to fear, to move'.
As, पाणिपादम्, 'the hand and foot' शिरोग्रीवं 'the head and neck' मार्दङ्गिकपाणविकम् 'players on the mridanga and panava (kinds of drums)' रथिकाश्वरोहम् 'the soldiers on chariot and horse.' वीणावादकपरिवादकस्, रथिकपादातम् ॥

This rule applies to cases of Samahara Dvandva or aggregate Dvandva Compounds only; and not to Itaretara Dvandva 2.2.29

In the Dvandva Compounds of animals such as elephants, horses, &c., the singular is optional; 2.4.12 The present rule also gives us an index as to where we must make Samahara Dvandva and where an Itaretara Dvandva. The Dvandva compounds of words signifying members of animal body, players or army, are always and solely Samahara Dvandva. The compounds of words like दधिपयसी 'curd and milk' 2.4.14 are purely Itaretara Dvandva and can never be Samahara Dvandva; while compounds of words denoting tree, animals &c., 2.4.12 are optionally either Samahara or Itaretara Dvandva.

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.