I think it might be a Eelgrass limpet.
Found while cleaning up "landscape"around my tent, most likely from dredge and fill operations in the 50s-60s. We didn't observe any live 'Opihi's.
Another oddity - I think I see tentacles on the lower left, so not a nudibranch or a flatworm. Internal shell, about 10 mm. @kati-romblon and Peri Paleracio
Shipwreck Beach, north shore of Lanai.