Ultra Maxima's are usually just a little more intense looking than 1st grades. There is no physical difference, only beauty. Someone once said these are prettier we should all charge more.
1st grade Maxima's are identical physically but are usually a more drab color, and accepted as less beautiful.
Electric Crocea's same thing as ultra max's. 1st grade crocea's are simply less striking.
Now there are Teardrop Max's and Crocea's which have 9 tear like markings) for which they are named. They also come in normal browns, and golds, or can come in very brilliant colors also.
Squamosa's can be drab brown or blue which seems to be very very rare.
Then there are ugly old hippopus clams. Which even at their best are not comparable to widely excepted beauty of max's and crocea's.
Derasa clams also, and Gigas (which get to be huge!!! like 4-6 ft huge)
Once again beauty really is in the eye of the beholder here. So buy what you like.
Rob