The segments of the =thorax= are free, and their number varies from two in _Agnostus_ (Fig. 146) to twenty-six in _Harpes_ (Fig. 150, A). In the Trilobites confined to the Cambrian period the number (except in the Agnostidae) is usually larger than in the genera found in the Ordovician and later periods. Owing to the free thoracic segments many Trilobites were able to curl up somewhat after the manner of a Wood-louse (Figs. 137, D, 138). The axial part of each thoracic segment is more or less considerably arched. Usually it consists of three parts: (i.) the largest part (Fig. 137, C, _a_), called the ring, is band-like in form, and is always visible whether the Trilobite is extended or coiled up; (ii.) in front of the ring is a depressed, groove-like part (Fig. 137, C, _b_) separating it from (iii.) the articular portion (_c_) which is convex in front and extends beneath the ring of the preceding segment; this part is only visible when the Trilobite is coiled up or when the segments are separated. In some few genera the axial part consists of a simple arched band without either a groove or a specially modified articular portion. The pleurae (Fig. 137, A, _l_, C, _d-f_) are fixed firmly to the axis, and have the form of narrow bands with the ends rounded, obtuse, pointed, or spinose. In a few cases the pleurae have a plain surface; but usually they possess either a ridge or a groove (Fig. 137, C, _g_); the former is generally parallel to the margins of the pleura, the latter is generally oblique, being inclined backwards from the axis. Sometimes in front of the ridge there is a small groove. On the ventral surface each pleura shows, at its outer extremity, a reflexed margin or doublure. At some distance from the axis the pleurae are bent downwards and backwards. The point where this bend occurs is called the “fulcrum” (_e_); it divides the pleura into an internal and an external part: the internal part (_d-e_) is flat or slightly convex, and just touches the front and back margins of the adjacent pleurae; the external part (_e-f_) may be (i.) narrower than the internal part, so that it is separated from the previous and succeeding pleurae; such occurs principally in pleurae with ridges, as in _Cheirurus_ and _Bronteus_; or (ii.) it may be in the form of a long cylindrical process, as in many species of _Acidaspis_; or (iii.) the external part may be of the same width, either throughout or in part, as the internal part, and may overlap the next pleura behind; this type is found principally in pleurae with a groove such as in _Phacops_, _Calymene_, _Sao_, _Asaphus_, _Ellipsocephalus_. In some Trilobites there is beyond the fulcrum a smooth, flat, triangular part at the front margin of the pleura; this part is known as the “facet,” and forms a surface articulating with the preceding segment which overlaps it. In the remarkable form _Deiphon_ (Fig. 151, E) the pleurae are separate throughout their entire length.