Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/258

 254 point out the specific uses of each minute variation, in the arrangement of parts fundamentally the same.

The geographical distribution of Ammonites in the ancient world, seems to have partaken of that universality, we find so common in the animals and vegetables of a former condition of our globe, and which differs so remarkably from the varied distribution that prevails among existing forms of organic life. We find, the same genera, and, in a few cases, the same species of Ammonites, in strata, apparently of the same age, not only throughout Europe, but also in distant regions of Asia, and of North and South America.

Hence we infer that during the Secondary and Transition periods a more general distribution of the same species, than exists at present, prevailed in regions of the world most remotely distant from one another.

An Ammonite, like a Nautilus, is composed of three essential parts: 1st. An external shell, usually of a fiat discoidal form, and having its surface strengthened and ornamented with ribs (see Pl. 35, and Pl. 37.) 2d. A series of internal air chambers formed by transversed plates, intersecting