Clam hitchhiker or abnormal growth?

[CaymanR]

It is hard, like the shell so at first I wrote it off as abnormal growth. It does seem to be growing... Is it some sort of barnacle? It doesn't look like it has another "half" or any sort of a body / mouth but it could go further into the clam.

 

[reef lover]

Looks like a thorny oyster.