Gary
I've been to south China and found that the Maximas there have very large scutes, some of them look very similar to Squamosas.
Been into Clams for the last years and been diving after clams in many locations in the world.
I think that in some places some species of clams have special characteristics that we can find only at that place, it might be that in some places, those clams are also hybrids.
For example if you look at page 123 in the Julian Sprung book Invertebrates A quick reference guide, you'll see what refered there as T.squamosa ? T.rosewateri ? this clam is quite common in the Red Sea and considered T. squamosa there.
I never seen a T.squamosa like this anywhere else in the world .
Another example, alot of Maxima clams from China has no elongate shell at all, they have the "rounded" symetric shell of the Squamosa, but the ribs are not symetric as the Squamosa is.
And have a small, but still larger byssal opening than the "regular" Suqamie.
Hybrid? maybe.