Page:Ann. Lyc. Nat. Hist. 11 155-162.djvu/6

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Ceraurus pleurexanthemus.

signed to be compared with the several figures in Plate XI, which have been drawn from the combined evidence obtained by the examination of numerous specimens.

It should be borne in mind, that as the shell, in the following description, is supposed to be placed back-downward, as shown in figure B, Plate XI, the words upper and under, etc., when used in this article, are to be taken in their strict sense, as compared with figure B, and not in the sense that they usually have, when a trilobite is placed with its back uppermost.

Head. Anterior, lateral, and free posterior margins bordered by a "doublure." Glabellar depression, concave, longer than broad, narrowed posteriorly; anterior margin a semi-lunate curve, to which the hypostoma is attached by a suture (hypostomatic suture); from the extremities of this suture, lateral ridges extend to the posterior lateral margins of the central neck depression. Four short processes project obliquely backward from each ridge into the glabellar depression. The four anterior processes are rudimentary and concealed by the hypostoma. The four posterior processes have rounded knob-like tubercles upon their upper extremities; the posterior pair attached opposite the inner posterior angles of the occipital depressions. A low arching ridge separates the glabellar and neck depressions.

The occipital depressions include the spaces within the "doublure," glabellar ridges, and neck depression. Occipital cavity in the anterior lateral third. Glabellar and occipital depressions finely punctate.

The neck depression extends laterally as shallow grooves under the "doublure," deepest towards the central depression. Central depression a concave groove, the posterior margin reflected upward and forward, terminating in a thin edge, which articulates with the articular fold of the first thoracic segment.

The facial sutures arise a little on each side of the centre of the posterior margin of the frontal "doublure, pass forward crossing the "doublure," and curve under its anterior margin, thence obliquely backward to the anterior margin of the occipital cavities, then into those, describing a curve around their anterior lateral bases, and passing out at their posterior lateral margins; thence they extend obliquely outward and backward to the lateral margins of the cephalic shield at their posterior third, obliquely cut the "doublure," and terminate at its inner margin, at the posterior lateral angles of the occipital depressions.

Hypostoma subovate, with wing-like extensions of the anterior lateral margins; central convex portion surrounded by a sinus, and an elevated margin; this margin, at the anterior half, widens, and forms a slightly ele-