Based on the pattern from OddKnits (www.oddknit.com/patterns/beach/cockleshell.html). Brachiopod shells have symmetric shells but one shell is deeper, or more convex, than the other. This is an attempt to make the deeper bottom shell (use the original pattern at link for top shell). Entered here to check stitch counts and rib alignment; I haven't checked to see if it knits up correctly.
Shown with a cast-on count of 0 stitches.