Each family has its own way of showing age of its shell.
Many families such as cones, terebras, continue to add to the lip and
grow in a more or less constant pattern.  The size can give a good guess
as to age in some species that have been well studied.  Frank Perron did
excellent papers on cones from Hawaii for instance.  Cone shells grow to
a mature size and then grow increasingly more slowly.

Some families grow a shell in one pattern while juvenile, and when fully
adult add a final varix and after that do not add to shell length.
Strombidae is a family where the final varix is primarily a great change
in the lip.  Lambis, for example grow spines on the final varix.  In
Cypraeidae, the adult shell adds a base on both columella and labial
sides leaving only a narrow aperture.  After this base is added, the
Cypraea adds only shell thickness (and pattern) with no major growth in
length.

Some bivalves grow the shell by addition to all the lip edges.  In the
process, the hinge also grows in length.  In some Arcidae, the teeth on
the hinge increase with age.  In some Arcidae species the number of
teeth is very linear with shell size.  Some authors differentiated
species by the number of teeth in Arcidae, but this is not a very good
criteria due to the number of teeth increasing with shell size.

In Dentalium, growth of the shell is to the anterior circular lip and
the shell grows as a narrow conic tube.

Some shells seal off the apex at a certain shell size and then lose the
apex above the seal.

Papers on growth is scattered througout the literature.  Most writers
deal with only one family, or one species.  Many books give a general
treatment on growth.  Does anyone have a book dealing with growth as the
major topic?
--
                     Aloha from Wesley M. Thorsson
Editor of Internet Hawaiian Shell News, a monthly Internet Publication
           122 Waialeale St, Honolulu, HI  96825-2020,  U.S.A
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