Going from memory ( I had a crocea until I pulled it off a rock too hard) a crocea was kind of rounder with softer edges.. a maxima has sharper more defined edges and are generally much smaller.
Pulling out Gresham's 'Giant Clams by Daniel Knop' book that I still have to give back *
*(sorry G!) heh.. it says the following.. Tridacna maxima:
"long, densly packed rows of scales only at the upper rim, the lower half of the shell is smooth."
Tridacna crocea:
"few long scales, shell with file resembling horizontal corrugations."
Umm. Yeah. How about some pics:
Maxima Crocea
hope that helps,
matt
difference of clams 
