What kind of bones is bone china made from?

Most customer ask me , what kind of bone is bone china made from ? Animal ? what's animal ? pig , cattle ? 
usually it's a mistake , the bone china is made of bone , it's not a really animal bone . 
It isn't made *from* bone, it looks *like* bone.

Apparently "Bone China" was first developed in Britain (Not China)
References list OX bone as a major source

Bone china uses about 40 to 50% bone ash in its content to strenghten , whiten and lower the firing temp 100 to 200 degrees. It appears that some modern bone china is made with calcium carbonate rather than bones as a source..