Recently I got my hands on this lot of 143 shells of Mesodon zaletus from Tennessee. According to the collector, the shells were representative of a single population. This was a good opportunity to look at the variabilities of various shell characteristics in this species.
Here are some of the things I noticed.
Shell height was quite variable. There were relatively flat shells and then there were shells with higher spires. The next photo shows 2 examples from the extreme ends of the spectrum.
The characteristic parietal tooth of the species was also variable in dimensions.
Two shells in the entire lot did not have parietal teeth. Here is one of them.
I don't know if the parietal tooth continues to grow throughout the life span of a snail. The toothless shell pictured above had a fully developed lip and so it was not a young adult. I suppose a rare mutation or some developmental abnormality may block the formation of the parietal tooth. Whatever the underlying cause is, it does not otherwise have lethal consequences, at least not always, for this particular snail became an adult. In one other species that I am familiar with, Neohelix albolabris, which is normally toothless, the opposite occurs: there are occasional specimens with a parietal tooth.