Hello Virgilio Liverani <[log in to unmask]>.
 
You wrote:
 
>[...]Some friends have problems in identificating a large ( 20 cm. and
>more ) Unio that spread a few years ago in eastern Padania plates, in
>slowly running waters . It is about trapezoidal in shape , with thin
>shell , and they always crack in the middle , regardless the way you
>treat or store them .
 
Must be Anodonta cygnea (L., 1758). The post mortem cracking of the
shell seems to be a typical feature of this species.
 
 
Arjan (79 years old, fat, bald, ugly and specialized in Stupididae)