Within a single study, the critical factor is consistency.  If you measure it the same way every time, and describe exactly what you did, that should cover most situations.  It might not hurt to measure the same specimen more than once to get an idea of the error range.

Although using a digital camera to photograph the specimen and then using various shape analysis programs to do the measuring has its appeal, it's critical to be sure that the focus is consistent and on the plane that you want to measure (usually the median axis of the shell).  Often, it's necessary to prop the shell into a flat position.  Clay, sand, and lead shot are among the popular supports for shells to get them into the right plane.  If you are including a scale bar in the photo for reference (useful in many analysis programs), be sure it is in the same plane, too.  Otherwise, the distorsion can be appreciable.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487-0345 USA
    [log in to unmask]

That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa

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