This is a real problem, as spires aren't perfect cones; and most
publications I've seen that use "angle of spire" don't provide an adequate
definition or discuss the question.

I suspect that the pragmatic answer is:

(a) where the sides of the spire are reasonably straight, people measure the
angle between them.  In practice, the tip of the protocone is usually
rounded and below the theoretical sharp apex of the cone, but there's no
easy way to take account of this and the difference is small enough to be
ignored.

(b) where the sides of the spire are less straight, a rougher estimate is
made on the basis of an imagined cone that starts at or just above the tip
of the protocone and contains the whole spire.

(c)  where the shape of the shell makes this clearly problematic, people
usually give up and don't try to measure the angle of spire.

Bas


----- Original Message -----
From: "Dr. Tilo Eggeling" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, June 28, 2004 11:15 PM
Subject: Measuring the apex...


> Dear friends and specialists,
>
> I am wondering whether there is an approved method for measuring the
"angle
> of spire". Which part is one to use for this measurement, just the
> protoconch or all the way down to the body whorl? And how significant is
> this data?
> Actually, since the apex is not a perfect cone, the measurement could also
> depend on "where" one measures this angle. How much would the values for
> this angle differ if measured holding the shell with the aperture pointing
> upwards and turned 90deg. respectively...?
>
> Any idea, comment or advise will be very welcome.
>
> Kind regards,
> Tilo
>
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