Measuring Shells

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Two methods of measuring shells: the classic & modern methods- - longer is better!

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by Richard L. Goldberg

A question came up a few days ago relative to the best and/or most accepted method for measuring shells accurately.  In other words, is there a standard method in which shell dimensions can be measured?  This assumes one is using calipers or a firm ruler and not a flexible tape measure that would result in a contour measurement.  It should be noted that for scientific purposes, multiple dimensions (width, height, and diameter) are often necessary to document a species, especially when describing a new species.  For a conchologist, one measurement usually suffices to document a specimen.

 

Gastropod shells have classically been measured perpendicular to the axis of the shell, meaning from the tip of the spire (posterior end) to the extreme end of the canal (anterior end).  This seems like a fair and legitimate method unless you are measuring shells such as Strombus where the outer extremity of the lip often far exceeds the height of the spire.  Over the past two decades with the advent of documenting so-called World Record Size shells (WRS) the classic method for measuring shells has been superseded with measurements using the “longest dimension”.

 

Measuring Trochid and Turbinid shaped shells presents other challenges.  Depending on the width of the base in comparison to the height of the shell (width-height ratio), sometimes measuring

from the tip of the spire to the outer edge of the lip yields the longest dimension; in other cases the tip of the spire to the center of the base is the longest

dimension.   Another challenge is when measuring shells with attachments such as Xenophora or spines like the Spondylus.  Classically measurements of only the shell were used whereas using the method of longest dimensions included the spines or attachments to establish the extremities.

 

If measuring the longest dimension of a shell and measured consistently from shell to shell and species to species one would assume the results are like comparing apples to apples and oranges to oranges.  Unfortunately with so many variables among the diversity of molluscan species, “longest dimension” measurements have yet to be fully adopted or standardized throughout our hobby.

 

The adjoining photograph illustrates a few examples including the classic method of measuring shells (white lines) along the axis or extremities of the shell and the longest dimension measurements (yellow lines).  No doubt this topic is open to varying opinions since beyond a WRS shell measurement standard, no other system has yet to be proposed or adopted. © worldwideconchology.com (May not be copied or reproduced in full or in part without the express written consent of the author).

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