During my survey of the land snails of the Monocacy Natural Resources Area, I once collected a large live snail that was a subadult of either Neohelix albolabris or Mesodon thyroidus. The adults are easy to tell apart, because N. albolabris has a completely sealed umbilicus, while that of M. thyroidus is only partially closed by the edge of its lip. The juvenile shells of both species have open umbilici, but they can be distinguished, with some difficulty, from their microsculpture. Rather than rely on its microsculpture to identify this snail, however, I decided to raise it to adulthood.
I collected the snail on the 19th of April. The 1st scan1 was obtained on 10 July. The shell diameter was 25.0 mm. The umbilicus was partially covered by the edge of the lip. The lip was thin and fragile as you tell from its condition at the time of the scan.
Only 4 days later, the snail’s shell had grown to a diameter of 26.3 mm and its umbilicus was now closed by a translucent callus with only a narrow slit left open. The umbilicus starts to close starting at the edge of the lip and the callus grows towards the far end of the umbilicus. So the slit, although not quite visible in the photo, was along the edge of the umbilicus farthest from the lip. The lip was still very fragile and parts of it broke off when I was handling the snail.
Finally, on 27 July, when the final2 shell diameter was 27.2 mm, the umbilicus was completely sealed and the lip was reflected and hardened. It was a N. albolabris.
1Back then I didn’t have a digital camera; these pictures were obtained with a flatbed scanner.
2Neohelix albolabris has determinate growth.