Ancient Roman statuary at the Metropolitan Museum of Art, New York: NEREIDA E TRITONE -- The Bothmer Purchase Fund, 1993.

Speranza

Marble lunette with Nereid riding Triton
Period: Mid-Imperial, Trajanic
Date: 1st quarter of 2nd century A.D.
Culture: Roman
Medium: Marble
Dimensions: Overall: 46 1/2 x 69in. (118.1 x 175.3cm)
Classification: Stone Sculpture
Credit Line: The Bothmer Purchase Fund, 1993
Accession Number: 1993.11.2
This artwork is currently on display in Gallery 162

This relief must have been one of a series that decorated a public building or perhaps a grand funerary monument.

The iconography derives from a passage in The Iliad that describes the Nereid Thetis, mother of the hero Achilles, and other Nereids carrying newly forged armor to her son.
References

Picon, C. A. 1993. "Recent Acquisitions: A Selection 1992-1993." The Metropolitan Museum of Art Bulletin 51(2): p. 16.

Picon, Carlos A., et al. 2007. Art of the Classical World in the Metropolitan Museum of Art. New York: Metropolitan Museum of Art, no. 425, pp. 366, 488.