Central Texas Fossil ID

[Boots]

Not a particularly exciting ID request, but these have been driving me crazy. I pick up tiny fragments of these in the North Sulphur River and Post Oak Creek, and picked up some bigger chunks while hunting in central Texas lately. On the "exterior" the striations run vertical, but on the cross sections the striations are horizontal. On larger chunks there are sometimes a layer of pocketed material on the back. Is it petrified wood, shell fragments, some type of enamel? I keep grabbing these things without knowing for sure what they are, and I'm ready to toss them if I can't figure it out.

 

http://imgur.com/a/qwKtl

 

Any help is appreciated. Thanks y'all!

[BobWill]

Welcome to the forum. These are broken pieces of bivalves. The cross-section shows where calcite crystals were formed. The pockets are probably where oysters attached to the clam shell.

[Fossildude19]

Posting the images here for continuity and for those who don't want to or can't access imgur. 

 

            

 

 

 

 

 

              

 

 

 

   

[PFOOLEY]

 

*image from http://www.springcreekforest.org/Fossils.htm