Crystallized Carbonate of Lime, lenticular-equiaxed

The figuring of this shell will not only serve a geological purpose, and show a curious crystallization, but help to explain the flattened crystals in the next plate, which are not easily understood, as the lines they form in the drawing give but little idea of flatness, and may seem to express the perspective of a cube, especially as we are not yet much accustomed to these representations.

This is the Helmintholithus Ammonites of Linn. Gmel. v. 3. 411. usually called Cornu-ammonis, of which there are many species found in the petrified state^*, abundant in many parts of Great Britain. Abroad they are often siliceous, or at least contain siliceous crystallizations; but in Great Britain are mostly calcareous, found in lime-stone  rocks and marly places. The shelly part may some of it be the remains of organic structure. The crystallized internal parts of shells and stones afford a curious subject for inquiry. In the chambers of this nautilus, (for so the living genus is called by Linnæus, see Gmel, v. 1. 3369.), the matter of crystallization may have passed through the alveolus, or little hole, to each partition. In other shells, and in geodes, it must be otherwise. The crystals are rough, and in nearly a regular series from the primitive to the equiaxe. The faces however of the latter are rounded, giving it a lenticular form. They are also somewhat striated, resembling the lenticular crystals of certain spathose iron ores.