A walrus tusk Ivory 'Okimono' of a skull and Frogs and Snake, (Height 5.7 cm) Late 19th century. Japan. Unidentified Muster.

Japanese Okimono 「置き物」means kind of a decorative art.
 This skull's okimono's Height 5.7 cm
Also, This Skull doesn't symbol of "MEMENTO-MORI" nor " Vanitas"