Hello,

I'm sorry I wasn't clear on the Sweep construction. The green lines were setup to be perpendicular to the curves, and the orange three point arc was Scaled and Aligned to the green lines seen here. Once the initial outer section was Swept, subsequent sections spiraling inward were Swept using the edge of the surface and the following curve. If you wanted to make the sink you would increase the size of the inner orange curves and decrease the size of the outside. If it's helpful I think I have some time to make a short video of the process. Keep in mind it's just one of a few different ways you can construct this model, you may come up with a more efficient way to do so.

Hello OSTexo,
my english is not so good - and I guess this is why I do not understand what your intention in this ammonite thread is?!
Do you want to make an ammonit for yourselves? Or are these suggestions in order to show me an alternative modeling concept?

Hello Michael and Pilou,
now I think the steps for going into the final ammonit modeling are clear to me!
Thanks a lot.
Lara

Hello,

Correct, an alternative modeling method.

Another method ;)
Log Spiral by Bemfarmer
http://moi3d.com/forum/index.php?webtag=MOI&msg=4543.1
+ Rail Revolve