Among the specimens that were presented to the British Museum by Dr. C. T. Trechmann in 1917 from the collection of his father, the late C. O. Treehmann, was a loose crystal~ which at first sight appeared to belong to one of the lead-grey species so long associated with the Binn Valley, but measurements made on the three-circle goniometer in the British Museum did not accord with the angles recorded for any of the species hitherto described. The crystal was recently submitted to a careful examination, but still the bi-angular co-ordinates of the faces differ from those given for any other species, and the only conclusion that can be drawn is that the crystal represents a new mineral species, and one peculiar to the Binn Valley.