Spotting

Description:

A friend of mine brought me this bone (found on the beach in Santa Cruz, California) in hope of me being able to identify it, but I haven't made an ounce of headway as to what it might be. It is 6.1 mm long, 3.4 mm wide (at the widest point), and extremely lightweight. Does anyone have any clue what creature this may belong to, and even what part of its anatomy it may be?

Habitat:

Found on the beach.