Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/263

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The bosses thus often introduced at the origin, division, and termination of the ribs, resemble those applied by architects to the intersections of the ribs in Gothic roofs, and are much more efficient in producing strength. These tubercles have the effect of little vaults or domes; and they are usually placed at those parts of the external shell, beneath which there is no immediate support from the internal transverse plates (see Pl. 37, Fig. S. Pl. 42, Fig. 3. c. d. e. and Pl. 40, Fig. 5.)