Arrow-head, or finger stone; vulgarly called thunder-bolt, or thunder stone. A genus of fossil shells, common in chalk and limestone. These shells consist of an interior cone, divided into partitions connected by a syphon, as in the nautilus, and surrounded by a number of concentric layers, made up of fibers radiating from the axis. These layers are somewhat transparent, and when burnt, rubbed or scraped, give the odor of rasped horn. The species are now extinct.

A conical calcareous fossil, tapering to a point at the lower extremity, with a conical cavity at the other end, where it is ordinarily broken; but when perfect it contains a small chambered cone, called the phragmocone, prolonged, on one side, into a delicate concave blade; the thunderstone. It is the internal shell of a cephalopod related to the sepia, and belonging to an extinct family. The belemnites are found in rocks of the Jurassic and Cretaceous ages.