Modern or fossil clam?

[KimTexan]

I’m not sure if the reference either. @Plax Yes, I believe the shells could be Pleistocene. The formation in the riverbed is Cretaceous. The banks have Pleistocene though. The list is of shells found in the river. What era they each are from I do not know. The book has numerous examples of Pleistocene fossils such as mammoth, ground sloth, ox, and horse to name a few. So it is conceivable that the list is not restricted to Cretaceous.

[DPS Ammonite]

Here's a list with photos of modern Texas freshwater clams. Now we need a reference of Pliocene/Pleistocene freshwater clams.

 

https://www.inaturalist.org/lists/66510-Texas-Freshwater-Musselss-Check-List?page=2

 

EDIT: I see now that others have mentioned Amblema plicata. Maybe reference above will help with all IDs.

 

See photo of Amblema plicata from Arkansas (occurs in Texas too) which looks a lot like one of your clams.

 

https://molluskconservation.org/FMCS/Workshops/2007/2007Symp/2007Saline.html

 

https://en.m.wikipedia.org/wiki/File:Amblema_plicata.jpg

[KimTexan]

That is helpful. Mine do fall among the modern clams, but @Fruitbat pointed out that he found the plicata in the Pleistocene layer at NSR under bison bones I believe it was.

The plicata that is in the 2nd link looks more like mine, but very different than the one in the 3rd link. I’m wondering if the one in the 2nd link is the correct ID. Maybe it is the Fourridge or Fiveridge rather than the Threeridge. The number of ridges on them appear to be different. Also, you can see more ridges on the edge of the shell whereas you cannot see that on the one in the 3rd link. Maybe it is the lighting or where they begin counting is different.  

 

I am not sure where we would find a list of Pliocene/Pleistocene freshwater clams for the area. I’m ok with them being modern clams though. I just thought they were possibly not modern since I had never seen anything like them in the area before.