Last bit of evidence.
Tiny, about 1/2" across carapace.
Dried hydroid colony on seagrass.
Leafy Hornmouth? The shell, not the hermit, please.
The shell, not the hermit, please.
Black tips on claws, though not visible in this photo. About 6" across.
The top was white (sorry for the glary photo), and the base was oddly flattened.