Toheroa

Paphies ventricosa, or toheroa, is a large bivalve mollusc of the family Mesodesmatidae, endemic to New Zealand. It is found in both the North and South Islands, but the main habitat is the west coast of the North Island. The best grounds are wide fine-sand beaches where there are extensive sand-dunes, enclosing freshwater, which percolates to the sea, there promoting the growth of diatoms and plankton generally.
This shell was spotted at Taylors mistake, and the photo was also uploaded in my personal blog.