Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/264

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Similar tubercles are introduced with the same advantage of adding Strength as well as Beauty in many other cognate genera of chambered shells. (Pl. 44, Fig. 9. 10. 14. 15.)

In all these cases, we recognise the exercise of Discretion and Economy in the midst of Abundance; distributing internal supports but sparingly, to parts which, from their external form, were already strong, and dispensing them abundantly beneath those parts only, which without them, would have been weak.

We find an infinity of variations in the form and sculpture of the external shell, and a not less beautiful variety in the methods of internal fortification, all adapted, with architectural advantage, to produce a combination of Ornament with Utility. The ribs also are variously multiplied, as the increasing space demands increased support; and are variously adorned and armed with domes and bosses, wherever there is need of more than ordinary strength.

The uses of the internal air chambers will best be understood by reference to our figures. Pl. 36 represents a longitudinal section of an Ammonite bisecting the transverse plates in the central line where their curvature is most simple. On each side of this line, the curvature of these plates become more complicated, until, at their termination in the external shell, they assume a beautifully sinuous, or foliated arrangement, resembling the edges of a parsley leaf, (Pl. 3S,) the uses of which, in resisting pressure, I shall further illustrate by the aid of graphic representations.

of the dorsal portion of this species. Between these three Reels, or ridges, are two depressions or dorsal furrows, and as these furrows form the weakest portion of the shell, a compensation is provided by conducting beneath them the denticulated edges of the transverse plates, so that they present long lines of resistance to external pressure.