Biologic evolution

It is a Minos shellfish that wrote before.
The photograph in the section of Minos shellfish's shell was taken
today.



It is understood that the structures of Minos shellfish's shell are
three steps.
The shell is a thick, heavy shellfish.
It is likely to have become three steps to thicken the shell.
It is shape that doesn't exist in the shell that has been taken a
picture up to now.