Are sea shells animal or terrum?
what about coral reefs?
In todays world we know both are produced by animals, would they have viewed it the same way back then? I can see shells being known to be animal. but reefs would just appear to be rocks that grew.
IMO, shells are animal, and coral reefs are terram.
What about a hermit crab living in a discarded piece of pottery? 
Clearly a terram faerie, up to no good
I would say that coral are terram, but it would have some interesting, and to many suprising effects, if you use animal magic on it. 
E.
Oooo. I like this approach!
-K!