Genus: Stygiochelys GAFFNEY & HIATT, 1971
Etymology: Stygi-, Latinized combining form of Styx, one of the three rivers of Hades, the underworld of Greek mythology, refereeing to the Hell Creek Formation, in which the type specimen was discovered; and Greek, chelys, "turtle."

Species: estesi GAFFNEY & HIATT, 1971
Etymology: In honor of Richard Estes.

Holotype: AMNH 2601

Locality: SW 1/4, NW 1/4, sec.1, T15N, R55E, near Glendive, Dawson County, Montana.

Horizon: Hell Creek Formation.

Biostratigraphy:

Age: Lancian age, upper Maastrichtian Stage, uppermost Senonian subepoch, Upper Gulf Epoch, Late Cretaceous.

Material: Skull.