maxima:the shell is more elongated front to back and shorter top to bottom, with pronounced scutes in tight rows all the way down to the base of the shell.
crocea: shell is more symmetrical (short front to back, high bottom to top). the shell can close completely and is relatively smooth, sometimes with scutes at the top edge of the shell.
look at the diff betw pic 3 the maxi and 4 crocea. the ridges tell it all.
btw join reef-geeks dot com. clammie experts there. bill has 24 i believe.
that's where the wording came from