oops that is not a good sign Don. It is also not a death sentence either, I have one right now that lost its foot and is doing very well. I would make sure the store you got it from knows aboout that. It was probibly damaged when they removed it from the holding tank. Just set the clam in and keep an eye on it. alot of times they can recover.
I think what you see there is the Byssal thread bunndle, not the foot of the clam. Maxima and Crocea sometime relaease the entire thread buddle after it was cut. you find that this is just a tendon with a bunch of thread on one end. This is not the byssal gland. Youe clam will be just fine. it will put out more thread to attach itself to a new place.
I was just going to add what Minh said. That looks to me to be the byssal thread bundle. Not the byssal gland as that would be a bad deal. My clam shead their threads all the time and regrow new ones. They are all very healthy and growing new shell constantly.
I think you are correct. Its been awhile now and my clam is doing fine. What was a hole is now filled with white and he has even moved off the rock I had him on. I just put him back last night after finding him laying on his side. I'm sure he will settle in and attatch on his own.