Two suggestions for you:
1. If you are thinking of owning clams, IMO, get yourself Daniel Knop's book "Giant Clams." I'm a big proponent of doing research before buying and there is no better book out there on the topic. You'll get both pictures and textual descriptions of what distinguishes one type of Tridacnid clam from another. You'll want to consider many things before making your choice - including eventual size of the clam, lighting you can provide, feeding requirements, etc. Some folks refer to Derasa and Squamosa clams as "easier" but when their needs are provided for, they outgrow many people's tanks within a relatively short period of time. Crocea and Maxima clams stay much smaller but require more intense lighting.
2. Poke about here on RC and on the Internet. Search on "Maxima clam", "Crocea Clam", "Derasa clam" etc. and you will see tons of pictures. The shell is one of the key differentiators, but after a while you'll make an excellent guess from mantle pattern as well.
At any rate, your success will increase dramatically if you are armed with good information. Don't count on the LFS giving you that unless you happen to have an above average on in your area.
Cheri
). Ok so how do u tell which clam is which? The shell? Mantel? 
Or are they all pretty much the same and the only way to tell is by its color? 
