Page:Walcott Cambrian Geology and Paleontology II.djvu/144

88 around the palpebral lobe, they extend downward and slightly forward a short distance and then with a broad sweep curve inward, cutting the anterior margin on a line with the outer edge of the palpebral lobe. Cranidium with a large glabella, concave frontal limb, relatively narrow fixed cheeks, and elongate, narrow postero-lateral limbs. Glabella moderately convex, with the sides gradually converging to its broadly rounded front margin; it is marked by four pairs of short furrows that penetrate obliquely inward and backward from the sides; the two posterior lobes outlined by the oblique furrows are roughly subtriangular in outline, the furrows penetrating nearly one-third of the distance toward the center. The second pair appears to be represented by rather prominent, slightly convex tubercles, and extends about one-fourth the distance across the glabella. Viewed with a transverse light, the second pair of lobes appears to be a forward extension of the posterior pair of lobes, since the furrows back of them are not quite so deep as the more oblique furrow just inside of the inner postero-lateral margin of the second pair of furrows; the third pair of lobes extends obliquely inward and backward about one-third of the distance across the glabella; the fourth pair is outlined posteriorly by a rather deep furrow that increases in width from the outer margin inward for a short distance so as to form a shallow, triangular area. The anterior margin of the fourth pair of glabellar lobes is just back of a pit which occurs on the side of the glabella opposite the inner end of the palpebral ridge. On very finely preserved specimens a narrow, gently arched ridge appears to represent the extension of the palpebral ridges on the fixed cheeks. There is also a transverse furrow just within the anterior margin of the glabella. The glabellar furrows and lobes described indicate that the glabella is formed by the union of five or possibly six of the original segments of one of the ancestral forms of this trilobite. The occipital ring is separated from the glabella by a furrow that is rather broad and deep on each side, and narrow, shallow, and arching slightly forward across the center; occipital ring slightly convex, broad across the central portions, narrowing and terminating directly in the line of the posterior intermarginal furrow of the fixed cheeks. Fixed cheeks about half as wide as the glabella, nearly flat within the palpebral lobe and ridge, and sloping gently down into the postero-lateral limb. They are interrupted in front by the strong palpebral ridges which extend backward from a point opposite the anterior pair of glabellar furrows