Large trough shell 70 - 95mm
Maybe these are all murchisoni, however many times I sort them.
" Mactra discors was not found in Pegasus Bay during dredge surveys in 1991... and 2008..." http://www.nabis.govt.nz/Documents/Annual%20distribution%20of%20Trough%20shell%20Lineage.pdf
Only Mactra discors recorded in New Brighton in 1929. "quite common"

NatureWatch NZ is entirely to blame for the state of this coffee table.

Spisula murchisoni - NZ EEZ checklist
Spisula murchisoni - WoRMS, synonym Mactra murchisoni not accepted
Mactrona murchisoni - Te Papa
Mactra murchisoni - NZ Mollusca
... must be royalty