The edge of the shell that surrounds the aperture is the lip (sometimes called the peristome). The morphology of the lip of the shell of an adult snail is an important taxonomic character. The lip could be the same thickness as the rest of the body whorl (below, left). In such cases, it is referred to as a simple lip. In many species, however, when the snail is nearing the end of its growth, the lip of its shell considerably thickens. Thickened lips usually flare out and are referred to as being reflected (below, right).
Anguispira fergusoni (left) and Mesodon thyroidus (right), both from Maryland.
In some species a rib forms inside the body whorl immediately behind the lip. An example of this is shown below.
Zonites casius from Turkey. There is a prominent rib within its lip.
Many species of snails thicken the lips of their shells as they mature reproductively. Once their lips thicken, such species stop growing further and are said to have determinate growth. In contrast, snails whose shells retain a simple lip after they mature may continue to grow as long as they live, although their growth rate seems to slow down as they get older. They are said to have indeterminate growth. It is difficult to tell when a snail with indeterminate growth becomes sexually mature without dissecting it.
Mesodon thyroidus has determinate growth, while Anguispira fergusoni has indeterminate growth. But, it is difficult to categorize the growth pattern of Zonites casius. Species like Z. casius sometimes resume growing after they have built a rib inside their lips.
To be continued...
Snail shell terminology 1
Snail shell terminology 2
Snail shell terminology 4