Easton's Bible Dictionary: M. G. Easton, M.A., D.D.
A nail; claw; hoof, (Heb.sheheleth; Ex. 30:34), a Latin word applied to the operculum, i.e., the claw or nail of the strombus or wing-shell, a univalve common in the Red Sea. The opercula of these shell-fish when burned emit a strong odour "like castoreum." This was an ingredient in the sacred incense.