Fossalized crustacean

[Big T-Rex Fossil hunter]

Anyone familiar with this rock. Long story short have river rock around my house and found these. Going to guess that they get these rocks from the Mississippi somewhere but I live where a once shallow sea was. More of them will be posted as well. And will have the same story but be different fossils. Any guesses on this particular specimen it is about 1 and 1/4 in long and the whole thing is about 1 and 1/2 in long. The bigger part of the shell is more faded so it might not look like the whole shell is 1 and 1/4 in long but it is. 

[Big T-Rex Fossil hunter]

This is the outline because with out being in person it is hard to see the faded part.

[Fossildude19]

Looks like a very worn gastropod.

And gastropods are invertebrates, but not crustaceans.
Can't say more than that.