Their body size is microscopic with a shell diameter of less than 1 cm.

The umbilicus is deep and covers 1/7 of the shell diameter.

The shell diameter is usually 35-50 mm (2 in) or even larger.

The umbilicus is quite wide, reaching about 1/3 of the shell diameter.

Maximum known shell diameter is around 180 mm.

The umbilicus is large (almost 25% of shell diameter).

The largest known specimen measured 160 mm in shell diameter.

Juveniles used in cage experiments may have an initial size of 8mm shell diameter.

This opening constitutes about 15% of the shell diameter at its widest point.

Its shell diameter was only a bit larger than an U.S. quarter.