LITHIDIA, in natural history, the name of a large class of fossils, including the flint and pebble kinds.

The lithidia are defined to be stones of a debased crystalline matter, covered by, and surrounded with, an opake crust, and frequently of great beauty, and considerable brightness within, though of but a slight degree of transparency, approaching to the nature of the semi pellucid gems, and like them found in not very large masses.