Summary: The following are the main points discussed in this paper:—

Judged merely from its morphological development, calaverite appears to crystallize in the monoclinic system, the axis of symmetry being parallel to the edge of the prism zone; but the indices which must be assigned to the faces on this hypothesis are exceedingly complex. The faces are, however, found to lie in zones, but these cannot be referred to a single space-lattice: in fact there appear to be in all five distinct lattices which are incongruent, but not independent; the relations between them are discussed above (pp. 140–1). Since the morphological development of the crystals cannot be referred to one lattice, calaverite cannot resemble in structure those transparent substances which display the so-called optical anomalies. The actual co-existence of two or more incongruent lattices would result in heterogeneity, and the only plausible hypothesis, by which we may explain the strange phenomena displayed by this mineral, seems to be that there is a close intermixture throughout the structure of arrangements corresponding to different lattices.

Four types of twin crystals are described, of which the fourth depends on a single crystal, and requires confirmation. In the case of two remarkable twins one individual may be derived from the other by means of successive half-turns about the twin axes of types 1 and 2; no trace of an intermediate individual is discernible. The crystal representing type 4 may be another instance of this double twinning.

The crystals contain little silver, and in chemical composition approximate to the formula AuTe₂.