Dear Art,

Hormotoma is classified with the slit shells because the shell has a short
slit or notch in the aperture. As the shell grew, the trailing end of the
slit was filled in, so the slit left an easily recognized "track"
(selenizone) in the shell. There's a good drawing of this in the Treatise on
Invertebrate Paleontology, part I (the letter, not the numeral), p. I292.
However, most specimens of Paleozoic gastropods are only internal molds
without any original shell preserved, so it's not surprising if you don't
see this on your specimens.

The shells were originally made of aragonite, a not-quite-stable form of
calcium carbonate that may last for millions of years, but typically
dissolves even where calcitic shells are preserved. (This is why oysters and
scallops can be well-preserved in Tertiary rocks where the snails and most
of the clams are dissolved away.) In-depth work on Paleozoic snails
generally needs specimens that were silicified early. These can sometimes be
freed from the surrounding limestone by placing the rock in a bath of weak
acid. It's a slow process and requires a fumehood, but it can yield some
exquisitely preserved specimens. For details, see the "Handbook of
Paleontological Techniques."

As to why Hormotoma became extinct without leaving any apparent descendants,
this is the usual case. Why should Hormotoma be any different?

All the best,
Andy

Andrew K. Rindsberg
Geological Survey of Alabama