What holds two clam shells together at the bottom?

In clams, the hinge is actually located on the clams back. Hence the term dorsal hinge of dorsal ligament. The ligament is comprised mainly of tensilium and resilium... Both are keratinized proteins, but I don't recall the exact role and make-up of each. Sorry I don't have my marine biology with me tonight, I'll try to remember and look this up again tomorrow.