Never having tried to do an oyster shell led to an awful lot of trial and error Teejay. I first drew my base size on what I thought might look like the shape. Then I pencil drew some random lines to try and get the texture/shadow effect. Then I used the brush called Jims Brush that I d/led from
HERE for smudging. At some point I also did a fair amount of color adjustment layers. This is a screen shot of the very early work on the first shell. After I got the base shell looking somewhat as I thought it might then I started duplicating and clipping. Thank you for your nice comment and if there is anyway I can help further you know where I am.

Thanks to you too Joy.