Description of the Interior Surface of the Dorsal Shell of Ceraurus pleurexanthemus, Green

by Charles Doolittle Walcott

XVIII.—Description of the Interior Surface of the Dorsal Shell of Ceraurus pleurexanthemus, Green.

ВY C. D. WALCOTT.

Read June 7, 1875.

This interesting species, which has been referred to in the preceding pages, has already been described by earlier writers,^[1] as regards the general features of its structure and the outer surface of its shell. In this article, therefore, I shall omit all detailed reference to any of these points, and confine the description, as closely as may be, to the inner, or ventral, surface of the dorsal shell. This description is designed 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 elevated 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.

EXPLANATION OF PLATE XI.

Figure A.	Section of thorax at fourth segment; enlarged to two diameters.
	a.	Axial groove.
	bb.	Axial processes.
	cc.	Thoracic pleuræ and triangular depressions.
	dd.	Circular cavities.
	ее.	Free pleuræ.
	gg.	Inner extremities of thoracic pleuræ.
Figure B.	Interior of the dorsal shell; enlarged to two diameters.
	1.	Hypostomatic suture.
	2.	Hypostoma.
	3.	"Doublure."
	4.	Occipital depression.
	5.	Occipital cavity.
	6,7.	Facial sutures cutting "doublure."
	8.	Glabellar depressions and processes.
	9.	Neck depression.
	10.	Spines of the head.
	a.	Axial groove.
	bb.	Axial processes.
	cc.	Triangular depressions.
	dd.	Circular cavities.
	ее.	Free pleuræ.
	f.	Crescent shaped surface on free pleuræ.
	m.	Elevated margin of the hypostoma.
	rr.	Axial processes.
	s.	Smooth crescent-shaped surface on "doublure."
	t.	Pygidium.
	x.	Oval openings.
	z.	"Doublure."
Figure C.	Longitudinal section at median line; enlarged to two diameters.
	1.	Hypostomatic suture.
	2.	Hypostoma.
	3.	"Doublure."
	4.	Head.
	5.	Thorax.
	6.	Pygidium.
Figure D.	Section of segment at median line; enlarged to five diameters.
	1.	Posterior reflected articular margin.
	2.	Outer surface of segment.
	3.	Union of articular fold and axial ring.
	4.	Articular fold.

CERAURUS PLEUREXANTHEMUS, GREEN.

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1. Green, Monograph of Trilobites, 1832, page 84. fig. X. Hall, Palæontology N. Y., vol. I, page 242.