(1) The range of variation in the external characters of the rhynchonellids examined is illustrated by means of graphs and scatter-diagrams.

(2) The three-variable scatter compares directly the proportions of thickness, breadth, and length, and defines the assemblage as adequately as several scatters which relate only pairs of variables. A close relationship is shown to exist between the three-variable scatter in which the positions of specimens are mathematically determined and the scatter in which the specimens are placed according to general appearance.

(3) The three-variable scatter, when considered with size-groups, indicates that with growth, breadth and thickness increase more rapidly than does length. All the scatters indicate that proportionate thickness Varies to a greater degree than does proportionate breadth or proportionate length.

(4) The sample, being random, is probably nearly representative of the goniorhynch rhynchonellid fauna of the Boueti Bed at Langton Herring. The stratigraphical value of the work, however, can only be proved by examination of assemblages from other localities and horizons.

(5) Since the scatters show that all gradations occur between the extreme variants, the forms behave as though belonging to a single homogenetic community. It is recognized that internal characters may in due course be found to show discontinuity in their variation, but on present evidence it appears that only one species is present.