Page:Ann. Lyc. Nat. Hist. 11 155-162.djvu/7

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Ceraurus pleurexanthemus.

vated projecting surface; outer surface granulated. Interior concave, the margin a reflected edge or "doublure." Anterior margin a semi-lunate curve, attached to the anterior margin of the glabellar depression by the hypostomatic suture.

Thorax. Each segment may be divided into three parts, viz.: 1, Axial groove; 2, Thoracic pleuræ; 3, Free pleuræ. The axial groove consists of the axial ring, anterior "articular fold," and a reflexed posterior articular margin. The "articular fold" rests upon the thin edge of the reflected posterior articular margin of the next anterior segment. The anterior margin of the "articular fold" describes a curve from the anterior lateral extremities of the axial ring, forward into the axial groove, nearly concealing the preceding axial ring. The anterior margin of the axial ring is thickened, as a base for the articular fold, and also as the base of a pair of processes extending from the lateral extremities obliquely backward one-fourth the distance across the axial groove. Each process is a plate-like projection, surmounted at its upper extremity by a small knob-like elevation.

Thoracic pleuræ of each segment divided by diagonal ridges into two triangular depressions upon each pleura, separated from the axial groove and circular cavities, by short transverse ridges. Circular cavities situated between the triangular depressions and the free pleuræ; they are deeper than the triangular depressions. Anterior and posterior margins of the pleuræ parallel.

The free pleuræ curve outward and backward, terminating in falcate extremities. The hollow interior of each opens into the thoracic cavity at the inner extremity, which has upon its upper margin a crescent-shaped surface or slight sulcus. The whole thorax narrows posteriorly.

Pygidium semicircular, concave, and surrounded by a strong "doublure," which has a smooth subcrescentiform surface upon each anterior lateral margin. Anterior lateral margins parallel to those of the posterior segment of the thorax. The articular fold rests upon the axial ring of the posterior segment. The pygidium is composed of four anchylosed segments; the anterior one, penetrating the "doublure" and lateral margins, is produced into long curved spines. Four pair of axial processes project into the axial depression; the anterior pair well developed, the posterior pair as rudimentary tubercles under the "doublure." Upon the posterior surface of the anterior anchylosed segment, there are two minute oval openings, one on each side of the median line, the longer axis extending obliquely upward and backward.

Formation and locality, upper third of the Trenton Limestone, Trenton Falls, Oneida Co., N. Y.