Need Help IDing Molluscs in South Dakota

[boof]

Found a shoreline on the Missouri River in southeastern South Dakota with some mollusc fossils. There were lots like the one on the left, but only one I could find like the one on the right. Can anyone identify these? 

 

They'd be from the Cretaceous period, right?

The fossil on the right is the size of a quarter. The ones on the left range from softball to golf ball.

[SailingAlongToo]

The one on the right appears to be an impression of a nautiloid.

[Miocene_Mason]

I agree with nautiloid (probably ammonite) on the right. These were alive during the Cretaceous, and I've heard of one song in South Dakota.

[boof]

Thanks for the replies! Does anyone have input on the fossils on the left?

[Miocene_Mason]

8 minutes ago, boof said:

Thanks for the replies! Does anyone have input on the fossils on the left?

Maybe something akin to inoceramus?

[Fossildude19]

I would agree. 

Some sort of Inoceramid.

@jpc @erose @BobWill @Uncle Siphuncle

 

[Uncle Siphuncle]

The bivalves appear to be in the Inoceramus spectrum.  The ammonite impression should be compared closely with Prionocyclus.  Have you looked at a geomap of the collecting area?  This stuff looks Upper Cretaceous to me.  Ammonite ID can be pinpointed much faster if you share geologic formation.

[PFOOLEY]

18 minutes ago, Uncle Siphuncle said:

...The ammonite impression should be compared closely with Prionocyclus...

...as well as Collignoniceras woollgari regulare.

[westcoast]

1 hour ago, WhodamanHD said:

Maybe something akin to inoceramus?

Yes, something along those lines. Certainly Bivalve.

[RJB]

Yup, some kind of ammonite and the clams,,, inoceramid.

 

RB

[Uncle Siphuncle]

So we thinkin' Turonian?

[jpc]

I am going with inoceramus and collignoniceras.