Aydin,
Previously, I would measure shells with the tips of the calipers.  This
method causes a lot of error.  I started butting the apertural side of the
shell against the "handle" of the calipers, with the 'arms' of the calipers
above (on apex) and below (resting on basal lip) the shell.  This
positioning kept the shell more steady and allowed for more reproducible
measuring, i.e. making sure the vertical axis of the shell was parallel to
the axis of the caliper 'handles' and perpendicular to the 'arms'.

However, some shell shapes do not allow the lower 'arm' to touch the basal
lip and still remain in standard position (vertical axis of shell in same
plane, more or less, as caliper 'handles').  To alleviate this problem I've
gently placed the shell in standard position in sand or small shotgun
pellets...then measured the shell without handling it at all.......

Perhaps you have tried these methods...If not, check 'em out.....Kurt

At 02:30 PM 11/9/00 -0500, you wrote:
>Dear Conch-L'ers,
>A problem I have had in my studies of some North American polygyrid land
>snails, for example, Neohelix, Triodopsis or Mesodon, is the accurate
>measurement of the height. The shape of a poygyrid shell makes it difficult
>to reproducibly position the shell between the jaws of calipers to measure
>its height. I will appreciate your suggestions, comments, ideas to make such
>measurements easier, while maintaining accuracy & reproducibility. Thanks.
>Aydin
>