OOPS!  That was stupid.  I meant that the Gonioceras is an Ammonite (oid) not
a Gastropod.  Of course it is a Mollusc.  Back to Geo 101 for me.
Thanks for correcting me.

"Andrew K. Rindsberg" wrote:

> OK, here's the "Scientific American" version.
>
> Cephalopods, including ammonoids and actinoceratoids, are mollusks. Living
> cephalopods include squid, octopus, and nautilus. Extinct representatives
> include the lovely spiral shells of the ammonites.
>
> Early cephalopods had relatively straight shells, like long cones with
> chambers inside. The chambers were separated by walls ("septa") and
> connected by a long tube ("siphuncle"). In some groups, the shell became
> coiled into a flat, symmetrical spiral (e.g., modern Nautilus). The septa
> also became very complexly and beautifully shaped in some groups (e.g.,
> ammonites). But in the Devonian, there were still plenty of straight,
> conical ("orthocone") cephalopods swimming in the sea.
>
> Oddly, some of the ammonites became UNcoiled, though they retained the
> complex septa. These relatively late forms are common in Cretaceous rocks
> and include genus Baculites.
>
> Andrew K. Rindsberg
> Geological Survey of Alabama