Genus: Hydralmosaurus WELLES, 1943
Etymology: Greek, hydralme, “salt water”, Greek, sauros, “lizard”: Salt water lizard.

Species: serpentinus (COPE, 1871) WELLES, 1943
Etymology:

Referred material:

= Styxosaurus browni WELLES, 1952
Etymology:


Referred material:

BROWN, 1904

Locality: Head of Hat Creek Basin, 18 miles, southwest of Edgemont, Fall River County, South Dakota.

Horizon: Sharon Springs Member, Pierre Shale.

Biostratigraphy:

Age: Upper Lower to Lower Middle Campanian Stage, Senonian Subepoch, Upper Gulf Epoch, Late Cretaceous
Note: Now believed to be from the Younger Pierre Shale (MARTIN, SCHUMACHER PARRIS & GRANDSTAFF, 1998)

Material:

AMNH 5803: Fragmentary skeleton with gastroliths.
Note: Pteranodon sp found in the stomach region along with a teleost fish, Scaphites and gastroliths (CICIMURRI & EVERHART, 2001).