अग्निनिचयः (Agninichaya) / The Accumulation of Sacred Fire & the Classification of Pitṛs by Time-Order
आर्तवाः पितरस्तस्मादृतवश्च पितामहाः / समेतास्तु प्रसूयन्ते प्रजाश्चैव प्रजापतेः
ārtavāḥ pitarastasmādṛtavaśca pitāmahāḥ / sametāstu prasūyante prajāścaiva prajāpateḥ
Therefore the ārtavas are called the fathers, and the ṛtus the grandfathers. When they come together, they bring forth the creatures of Prajāpati.