Ok,

I realize there are not many ammonite t-shirts, but in case you are in need of a really hip present for that girl in your life...

http://www.cafepress.com/emilydamstra.69814806

Wow, I just searched ammonite, and look at how much cool stuff they have!

http://www.cafepress.com/buy/ammonite/-/cfpt2_/copt_/cfpt_/source_searchBox/x_0/y_0

Wow, that's excellent. I think the logo reads that it is Tornoceras, a Devonian ammonoid known from New York. Here's a nice close up of the superb illustration for you:

http://www.science-art.com/image.asp?id=1594&search=1