On the bottom (dorsal) side of a shell is its umbilicus. The shape and the diameter of the umbilicus is usually a species-specific character.
The umbilicus may be completely open (below, left), or partially covered by the lip (below, center), in which case it is referred to as rimate, or completely covered by the lip (below, right).
The shells of Haplotrema concavum (left), Mesodon thyroidus (center) and Neohelix albolabris (right). (Shells are not to scale.)
The umbilici of juvenile shells are always open. In N. albolabris the umbilicus is gradually covered by the edge of the lip as the snail matures.
To be continued...
Snail shell terminology 1
Snail shell terminology 3
Snail shell terminology 4