These are seashells – murexes from the deep waters off Vanuatu called Scabrotrophon inspiratum. Belgian researcher Roland Houart wrote about them (and as far as I can tell photographed them) in The Nautilus journal, Vol. 117, No. 3, published in 2003.
I like murexes because they’re so symmetrical on the one hand and so baroque on the other. Whorls of irregular spines, spiraling.