Taittiriya Samhita 7.1.5, Taittiriya Brahmana 1.1.3 and Taittiriya Aranyaka 1.23
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The Samhita: all was water; Prajapati moved on it as wind, saw the earth, became a boar and seized her, wiped her as Vishvakarman, and she spread out. The Brahmana: he sees a lotus leaf, infers something beneath, dives as a boar, brings up a piece of earth and spreads it on the leaf, fixing it with gravel. The Aranyaka: born alone on a lotus leaf, he shakes his body; sages arise from it and its moisture becomes a tortoise, which denies being his product and says it was there before; it rises as a thousand-headed person and, at his bidding, sets down water in each quarter from which sun, fire, wind and Indra arise.
The earth was once only a span wide; a boar named Emusha raised her, and her lord is Prajapati. Separately: Prajapati took the form of a tortoise (kurma) to create living beings, which is why all creatures are called children of Kashyapa; a live tortoise is laid in the altar, its shells the earth and the sky.
What is different here:
The boar has a name, Emusha, and is not said in so many words to be Prajapati; the tortoise is Prajapati himself and no rival.
At the start of the present age Brahma, who is Narayana, wakes and finds the world void and flooded. Inferring that the earth lies under the water he takes the form of a boar made of the Vedas and the sacrifice, as he had been fish and tortoise before. The Earth hymns him and asks to be raised as of old. He lifts her on his tusks; sages shelter in his bristles and praise each limb as part of the sacrifice. He sets her on the waters like a ship, levels her and piles up the mountains.
What is different here:
The diver is Narayana-Vishnu in Brahma's form, though still styled Prajapati; cosmic scale; the Earth is a goddess who speaks; hymns of praise; no lotus leaf.
All was water and the earth was formed in it; then Brahma the self-existent arose with the gods and, becoming a boar, raised up the earth. Muir notes that a later recension alters the line to identify this Brahma with Vishnu.
What is different here:
A halfway stage: the boar is Brahma, not yet Vishnu. Read only in Muir's citation, not in a Ramayana translation.
Sources: The Veda of the Black Yajus School (Taittiriya Sanhita), tr. A. B. Keith (1914); Original Sanskrit Texts, vol. 1 (2nd ed.), J. Muir; The Satapatha-Brahmana, tr. Julius Eggeling (Sacred Books of the East 12, 41); The Vishnu Purana, tr. H. H. Wilson (1840)