A reader in Botswana (yes, folks, Snail's Tales is read all over the world) left a comment at an old post and inquired about measuring snail shells. Here are some pointers.
Drawing from Kerney & Cameron, 1979. A Field Guide to the Land Snails of Britain and North-west Europe.
The height (or length, H in the drawing) of a snail shell is measured between the tip of the apex and the edge of the bottom lip. If the shell has a reflected lip, the measurements may be taken from right behind and below the reflected lip, excluding the reflected edge, but it is usually difficult to position the jaw of the calipers on the slope behind the lip reproducibly.
The diameter (or breath or width, B in the drawing above) is measured between the points indicated in the drawing above. But when the lip is sticking out too far, it could get difficult to position a shell between the jaws of calipers so that the axis will be parallel to the jaws. In such cases, I take the oblique diameter across the first suture above the aperture (the suture between the ultimate and penultimate whorls). This is marked by the red arrows in the photo below. This is not a standard measurement, however, and must be explained in a publication.
Certain types of shells, for example that of Helix aspersa, have 2 possible diameters. I discussed that in this paper.