Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/323

 Rh variations both in the external and internal form and dimensions of each part The body (Pl. 49, Pig. 1) is supported by a long vertebral column attached to the ground by an enlargement of its base (Pl. 49, Fig 2.) It is composed of many cylindrical thick joints, articulating firmly with each other, and having a central aperture, like the spinal canal in the vertebra of a quadruped, through which a small alimentary cavity descends from the stomach to the base of the column, Pl. 49, Fig. 4, 6, 8, 10. The form of the column nearest the base is the strongest possible, viz. cylindrical. This column is interrupted, at intervals, which become more frequent as it advances upwards, by joints of wider diameter and of a globular depressed form (Pl. 49, Fig. 1, and Figs. 3, 4, a, a, a, a.) Near the summit of the column, (Pl. 49, Figs. 3, 4,) a series of thin joints, c, c, c, is placed next above and below each largest joint, and between these two thin joints, there is introduced a third series, b, b, b, of an intermediate size. The use of these variations in the size of the interpolated joints was to give increased flexibility to that part of the column, which being nearest to its summit required the greatest power of flexion.

At Plate 49, Figs. 6, 8, 10, are vertical sections of the columnar joints 5, 7, 9, taken near the base; and show the internal cavity of the column, to be arranged in a series of double hollow cones, like the inter vertebral cavities in the back of a fish, and to be, like them, subsidiary to the flexion of the column; they probably also formed a reservoir for containing the nutritious fluids of the animals.

The various kinds of Screw stone so frequent in the chert of Derbyshire, and generally in the Transition Limestone, are casts of the internal cavities of the columns of other species of Encrinites, in which the cones are usually more compressed than in the column of the E. moniliformis. The varieties of form and contrivance which occur in the column of a single specie of Encrinite, may serve as an example of analogous arrangements in the columns of other species of the family of Crinoïdeans, (see Pl. 47. Figs. 1, 2, 5, and Pl. 49. Fig. 4 to Fig. 17.)