22 x 22 x 6.5mm. Not common here.

Recorded in Lyttelton harbour 1974:-
http://docs.niwa.co.nz/library/public/CHC2003-045.pdf
"Ostrea-Sigapatella community: Small, dispersed patches of the oysters (Ostrea heffordi) and associated slipper limpet (Sigapatella novaezealandiae) were present wherever there were hard surfaces for settlement. This assemblage was more restricted than the others, its only other member being the small crab Halicarcinus whitei."
Ostrea heffordi = chilensis