Hi Paul;
thanks for the segnalation; I have immediately taken 5 shells that I
believe(d) to be tigris, used the caliper and the rock & roll trial.
Number 1 (I thought that it was a tigris, but...): 1,566 and rolls over onto
it's base
number 2 ( I WAS sure it was a tigris):  1,428 and rolls over onto it's base
number 3 ( I WAS sure it was a tigris):  1,377 and rolls over onto it's base
number 4 ( I WAS sure it was a tigris):  1,466 and rolls over onto it's base
number 5 (juvenile tigris) : 1,438 and rocks but stays on its back
 
So, I have tryed to keep the shell not perfectly parallel to the surface,
and # 2, 3and 4 stood on their back, # 1 no.
Is this the trick, how do you keep the shell? I have deducted that if you
put only the very top of the back on the table, the shell have enough
kinetic energy to roll over the same: am I right?
Sandro