Whilst determining some crystals of proustite in the Mineralogical Museum at Cambridge, and which came from Bolivia or some other part of South America, my attention was attracted by faces of trigonal pyramids, which, as shown in figs. 1 and 2, are the most conspicuous on the crystals. On measurement, these faces were found to correspond very nearly to those of the rhombohedra {73̄3̄} = 10 R (fig. 1) and {94̄4̄} = 13R (fig. 2). The figures represent with fair accuracy the relatively large development of the faces of these new forms. Another form, {11.7̄.7̄} = –6 R, represented by a single face on the lower end of one crystal, is also new for proustite.