What fossil is this? SD Pierre shale

[Slow Walker]

Is this a fossil of some sort? There is like shells pieces on the sides. Sorry for poor pics was trying to beat night fall. About 1 ft long rock. Thanks

[jpc]

Possibly an Inoceramus clamshell.  Looking at the last photo, are the edges of those individual brown plates made up of crystal structures that go perpendicular to the surfaces of the brown plates?

[Slow Walker]

9 hours ago, jpc said:

Possibly an Inoceramus clamshell.  Looking at the last photo, are the edges of those individual brown plates made up of crystal structures that go perpendicular to the surfaces of the brown plates?

It is much bigger than the clam shell fossils there. 

There are cracks across it. Like the fossil shrunk or the situ expanded. I don't remember any crystals. I didn't take it with me. 

[Mahnmut]

3 hours ago, Slow Walker said:

It is much bigger than the clam shell fossils there. 

There are cracks across it. Like the fossil shrunk or the situ expanded. I don't remember any crystals. I didn't take it with me. 

Inoceramus spp. grew up to a meter. They may occur together with smaller clams.

 

[jpc]

3 feet wide (a meter) and the shell can be 1/8 inch thick and more.