This mineral, as described by the author about two years ago in the London and Edinburgh Philosophical Journal, presents a new and very interesting example of the principle known to crystallographers by the name of dimorphism; having the same chemical constitution as the well known Baryto Calcite of Brooke, but having a form belonging to an entirely different system of crystallization. In Dr Thomson's System of Mineralogy lately published, however, it is described as a mineral species before unknown, having a composition different from that of every known mineral, and is therefore distinguished by the name of the Bicalcareo carbonate of Baryta.