Optionally dat is substituted for danta in a bahuvrīhi samāsa when it is preceded by the words śyāva and aroka,
Thus श्यावदन्तः or श्यावदन्, अरोकदन्तः or अरोकदन् ॥ अरोक means 'dark' 'non-luminous'. The word संज्ञायाम् of the last sutra governs this also, so that
when the compound is not a Name, the substitution does not take place; as श्यावदन्तः, अरोकदन्तः ॥,