bination with the planes of the primitive form, and with those of the first and second modifications in fig. 174. I regret the not having been able to ascertain the incidence of the planes of this on those of any other modification.
All the specimens on which I have noticed the planes of this modification, were long since brought from Cornwall, but from what mine it is impossible now to ascertain.
The twelfth modification consists in a decrease on those edges of the primitive crystal which are formed by the meeting of the two pyramids base to base, by which each edge is replaced by two planes, placed on the primitive faces, but inclining on the axis, passing through the edges, fig. 182. Pl. 23. By fig. 183, the planes of this modification are represented in combination with the secondary pyramid, as will be evident on consulting fig. 184, which, together with fig. 185, represents the only crystals on which I have noticed the planes of this modification. I have not been able to ascertain their incidence on those of any other.
Most of the crystals delineated in the series annexed to this paper, are defined with great neatness and beauty: but there is generally much seeming confusion among the crystals of the oxyd of tin, arising principally from a circumstance or law, not altogether peculiar to it, by which similar portions of two or more crystals are regularly united, so as to form what have been termed macles, one of which has been described by De Lille by that name, and by Haüy