Fossil or Modern Oyster?

[FLRockHntr]

So I’ve been out of commission for some time due to back surgery. Just starting to work my way back into my favorite hobby. 
While cutting the grass on my property I found a recently exposed chert outcropping. After doing a little digging I found these two shells that were in a small area along with some chert, chert with agatized coral, and some beautiful colored chert. I have a small freshwater creek that runs through my property and this was within maybe 20 feet of it. They look like fossils, but I’ve been wrong before.

Thanks in advance and I look forward to starting to plan my next expedition!

[FLRockHntr]

After reading my post I’d like to clarify what I’m searching for. These shells were found with a large qty of dense high quality chert, solid agate and what appears to be worked material. This is on my personal private property and I guess the question is not is it a ‘fossil’ but shells from the same time period as what the other items appear to be. I’m not able to find any modern oysters with the same shell profile. I’m 40 miles inland from any salt water and there were about a foot or more deep underneath clay and sitting in a dark sandy material along with the chert near a freshwater creek that I own rights to (non navigable). Any help would be greatly appreciated.