Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/368

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This modification is represented by fig. 19. Pl. 16. and consists in a decrease on the four lateral solid angles of the primitive form, by which each is replaced by a plane, perpendicular to the axis passing through those angles.

Fig. 20. shews this modification in a more advanced state, and has been added not because it has been thus observed, but in order that the combination of the planes of this modification with those of the primitive form, may be the more readily traced in fig. 21. in which it occurs, though but rarely. In the fine collection of tins, in the possession of Mr. Sowerby, there is a specimen of