Yesterday, when I first saw this small snail (shell diameter ~5 mm) in a shallow depression on the trunk of a beech tree, I thought it was a Zonitoides arboreus. Under the magnifying glass, however, I noticed that its domed spire was more like that of Ventridens suppressus, another common land snail in the area.
At home, under the microscope, I could see the teeth inside the aperture, especially, the long outer-basal one (yellow arrow), which confirmed the identification as Ventridens suppressus.
The red arrow in the right-hand picture is pointing at the columellar tooth. In Pilsbry's (1946) terminology, this particular individual is in the 3rd or final neanic stage. During the maturation of the snail, the outer-basal tooth is resorbed; the adult snails retain only the columellar.
In most other snail species, the apertural teeth and laminae, when they are present, begin to develop as a snail approaches maturity. Ventridens suppressus is backwards in that respect; the earliest juvenile stages have even more teeth, up to 5, in their apertures. Their function is unknown.