Big shell! Old Shell! Help! Found in Montana

[KrattigerS]

Hello all! So about 10 years ago I was walking around in literally the middle of nowhere Montana on my friends ranch when I noticed something white sticking out of the ground. I walked over and thought first it was a rock, but soon realized it was the tip top of a shell  sticking out of the dirt. Very confused on how this shell got to the middle of Montana and in a field!!! (I know realize how it did)

But anyways I have no idea what it is so thought you all could give me some information on it. I've carried it around for a long I thought maybe a collector might be interested. It is about 12inches by 6 inches.

Pictures are attached!

Thanks,
Seth