Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/300

 296 analogies of structure, which place them in near approximation to the inhabitants of the existing seas.

The anterior segment of the Trilobites (Pl. 46, a, passim,) is composed of a large semi-circular, or crescent-shaped shield, succeeded by an abdomen, or body (c,) composed of numerous segments folding over each other, like those in a Lobster's tail, and generally divided by two longitudinal furrows into three ranges of lobes, from which they have derived the name of Trilobites. Behind this body, in many species, is placed a triangular or semi-lunar tail or post-abdomen (d,) less distinctly lobated than the body. One of these Genera, the Calymene, has the property of rolling itself up into a ball like a common Wood-Louse. (See Pl. 46, Figs. 1, 3, 4, 5.)

The nearest approach among living animals to the external form of Trilobites is that afforded by the genus Serolis in the class Crustacea. (See Pl. 45, Figs. 6, 7.) The most striking difference between this animal, and the