Page:Walcott Cambrian Geology and Paleontology II.djvu/145

NO. 4 and merge into the arched palpebral lobes; in front of the palpebral ridge the cheeks descend rapidly and merge into the frontal limb; the postero-lateral limb is long and almost entirely made up of the rounded, narrow posterior rim, strong intermarginal furrow, and a narrow area between the furrow and the facial suture. Palpebral lobe of medium length; in a cephalon 18 mm. in length it has a length of 4 mm.; it is narrow and raised a little above the surface of the fixed cheeks. Frontal limb broad and strongly concave in large cephalons, becoming less so in smaller; in front of the glabella it slopes rather rapidly downward and then upward with a gentle curve to the thin, rounded edge. Free cheeks gently convex, but become slightly concave toward the outer margin in the larger specimens. The genal angles are extended into short spines.

Thorax with eleven nearly transverse segments; axial lobe convex and a little more than one-half the width of the pleural lobes. It is strongly defined by a sharp angle where the segments pass into the pleural lobes. Each segment arches forward slightly at the center, also at the sides just before joining the pleural portion of the segment; the pleural lobe of each segment is transverse for about one-half its length, and then it curves gently down toward the falcate termination. The pleural grooves start just within the front rim of each segment next to the axial lobe, and, widening, continue with a nearly uniform width for about two-thirds of the distance outward, and then narrow, disappearing some little distance within the termination of the segment.

Pygidium large, moderately convex, with a narrow, strongly convex axial lobe and broad, slightly flattened margin; axial lobe five-sixths of the length of the pygidium, convex and narrowing gradually from the anterior margin to its termination; it is crossed by from five to six shallow, transverse furrows that outline from six to seven slightly convex rings and a terminal section; from the axial lobe the surface of the pleural lobes slopes at first gently and then quite rapidly down for about two-thirds of their width and then very gradually out to the margin. They are marked by five or six broad furrows and rather broad, flattened ridges that are the continuation on the pleural lobe of the transverse rings on the axial lobe; the posterior margin curves more or less inward toward the median line so as to give a more or less indented outline to the otherwise rounded curve of the sides and posterior margin of the pygidium.

Surface under a strong lens shows numerous pits or pores; the free cheeks and the cranidium in front of the glabella and palpebral