a marine fossil, of frequent occurrence in the chalk of the south of England and the flint of the north of Ireland. It is generally of a cylindrical or conical form, one end terminating in a point; the other, when perfect, in a conical cavity. It varies in size from that of a goose-quill to twelve or fourteen inches in length, and one to two in breadth. Its composition, whether enclosed in limestone, flint, clay, or sandstone, is uniformly crystallized carbonate of lime, striated, and radiating to the circumference from a line which passes perpendicularly through the fossil. Though distinguished by naturalists into several species, its nature and functions are comparatively little known. By some it has been considered a portion of an animal; but to what species it belonged no conjecture can now be formed. The appellation of thunder-bolt has been vulgarly applied to it, probably from some fanciful analogy in point of form.