Curious Oyster?

[RCD]

Someone else mentioned finding oyster fossils in the SF East Bay area CA, What you you guys think of this one? Area is a creek bed draining a diverse ranging from Miocene to Jurassic and in between. Thanks in advance for lending your experience and knowledge to hep me I.D. fossils. 

[Plantguy]

Interesting piece. Whats the rock's hardness? Does it react/fiz to diute vinegar/hcl?  

 

Maybe its a fossil oyster but I'm thinking non oyster/not a fossil but rather some thin banding/beds/layering within a piece of limestone or maybe banded chert if its siliceous. 

 

Regards, Chris 

[RCD]

Went back for this today. Definitely Oyster  And my son found another one in the same place a few feet away. I know they are oysters because I have shucked a lot of oysters in my day, and there is a certain morphology to the "rear end" (a little raisted/upturned) and overall look that is pretty unmistakable. I'll be posting more pics of this and our other finds in the fossil hunting trips forum since I no longer need an I.D. Cheers!!!