Gastropods?

[PaleoOrdo]

In the Kalvsjøen formation in the Oslo field I found these two stones. I wonder if they are gastropods or something else? The formation is of a rawthyean age late Ordovicium, the last period of Katian, before the Hirnation extinctions.

Next fossil:

 

[Planko]

I am no expert but I think they are oysters or some other bivalve

 

[PaleoOrdo]

Thanks for your suggestion. It cannot be brachiopod, and I wonder if bivalve have so many layers of exoskeleton?

[Myrmica]

This may be something like a stromatoporoid, i.e., sponge-like animals that had concentric, porous 'walls' of calcium carbonate.  

[PaleoOrdo]

I agree with stromatoporoids, as they are known to have many different shapes. In these cases it is not possible see the distinguishing marks, as a bulbous, bumpy surface. It is, however, possible to see another mark: some layered structures. The overall shape look similar to some kind of gastropod, so it may be enveloped by the sponge. It is known in general that sometimes shelled organisms could become enveloped entirely by the growing stromatoporoid.