(from βελενίτης, a dart), vulgarly called thunder-stone, a marine fossil, common in the chalk and lime formations. The fossil shells of this genus are of a long conical shape, the interior cone being divided into partitions connected by a siphon (as in the Nautilus), and surrounded by numerous concentric layers composed of fibres radiating from the axis. This genus, of which there are many species, belongs to the order Cephalopoda. See MOLLUSCA.