Summary: Three of the members of the plagionite group, fülöppite Pb₃S₈S₁₅, plagionite Pb₅Sb₈S₁₇, and semseyite Pb₉Sb₈S₂₁, show linear correlations of cell volume and density versus PbS:Sb₂S₃ mol ratios. This relationship can be used better to define the nature of the fourth member of the group, heteromorphite, Pb₇Sb₈S₁₉. The cell dimensions derived for heteromorphite are a 13·60, b 11·93, c 21·22 Å, β 90° 50′.