The protoconch of a snail shell is the apical portion of the shell that forms while the snail is still an embryo. In many species, the protoconch has a characteristic and species-specific morphology different than that of the teleoconch, the rest of the shell. However, it is not always easy to tell where the protoconch ends and the teleoconch begins.
Until tonight I had examined hundreds of shells of the land snail formerly known as Helix aspersa*, but not noticed how distinct the end of its protoconch was. The realization came to me when I happened to look at the following shell under the microscope.
This shell is an exception in that the demarcation between the protoconch and the teleoconch (arrow) is very distinct. Here is another and probably more typical shell.
Also notice that the surface of the protoconch is much smoother than that of the teleoconch.
*Now called Cornu aspersum, etc.