They looked like something between oysters and cultivated scallops common at wet markets. The shells are rather rugged and hard They had to use a hammer to crack the shells. The meat was sold at HK$ 150 for a pound.
I think they could be 車蠔 (spiny oysters, thorny oysters). Or rock scallops. I am not quite sure.
My wife stir-fried them. The meat is tasty but slightly tougher than the cultivated scallops commonly available from wet markets.