It's odd. There are 3D specimens that have weathered out, in other people's collections (this is my only example from that spot) - but I have seen 2 different IDs on them in the same display case!
I am curious what ID you have seen for the roundish ammonites. It is possible that there were two different genera in this diplay case which look only for you identical. Globose(roundish/sphaerish) ammonoids are very hard to ID. In Tuvalian time it can be Arcestes, Bacchites, Isculites or Indonesites. Didymites occurs later, it has a short occurence in Norian time/Alaun 1/ bicrenatus zone.
When shell is missing or preservation is bad, the suture line of the ammonoid is the best way to get an ID.
The suture pattern(Lobelines) are only visible at the surface of the inner core(phragmocone). Try to polish the surface. Then you can compare the lobes with the above named genera. Please do not grind to deep because the deeper you grind the more simply the lobes look.
@ Ed, IDing was tough for me too when I started with triassic ammonoids. Be sure that there are specialists out there with the same problems.
The main problem is that Triassic ammonoids are scarce to see/find and that there is a bunch of older literature/names all over the world that nobody will review because it is too much work.
Thank you for your nice comment in my gallery!
Eric, just read your recent post. I only know a little about upper Triassic ammonoids. This is maybe one third of all Triassic ammonoids. And from this third mostly the tethyan ammonoids and from the tethyan ammonoid only the ammonoids of the Alps. This is far away from good knowledge
Edited by andreas, 03 June 2014 - 02:58 PM.