DESCRIPTION OF GENERA AND SPECIES. 223 



This species appears to be most nearly related to Illocnurus eurekensis Walcott 

 [18846, p. 97, plate 12, figs. 4, 41;], which occurs at the base of the Ordovician in the 

 Eureka district of Nevada, but it differs in the smaller palpebral lobes, which are 

 situated farther back on the cephalon; and it differs from /. ceres [plate 23, fig, 4] 

 in its proportionately longer cephalon. 



The species is widely distributed in the Upper Cambrian limestone. 



Formation and Locality. Upper Cambrian : (0 3ft) Crystalline limestone near the 

 base of the Ch'au-mi-tien limestone [Blackwelder, 190711, p. 36 (part of the first list 

 of fossils)], at Ch'au-mi-tien; (C34) purplish-gray limestone about 100 feet (30111.) 

 above the base of the Ch'au-mi-tien formation [idem], in road at northeastern corner 

 of small village near Ch'au-mi-tien; (C 12) in the central part of the Ch'au-mi-tien 

 limestone [idem (second list of fossils)], i.S miles (2.9 km.) west-southwest from the 

 temple on Tsing-lung-shan. 7.5 miles (12 km.) east of Ch'au-mi-tien, and (C41) 

 lower part of the Ch'au-mi-tien limestone [idem (part of the first list of fossils)], 2.7 

 miles (^.3 km.) southwest of Ch'au-mi-tien, Shan-tung; also (C54), lower part of 

 Ch'au-mi-tien limestone [idem, p. 42 (part of last list of fossils)], near top of lime- 

 stone knoll two-thirds of a mile (i.i km.) west of Tsi-nan, Shan-tung, China. 



Collected by Eliot Blackwelder. 



Illsenurus ceres Walcott. 



Plate 23, Figures 4, 40. 



Ilktnunts ceres WALCOTT, 1905, Proc. U. S. Nat. Mus., vol. xxix, p. 97. (Described as a new species 

 essentially as below.) 



Cephalon, exclusive of the free cheeks, rounded subquadratc, moderately con- 

 vex; the posterior margin of the cephalon curves slightly upward opposite the 

 glabella, where there is a slight thickening which gives the appearance of a narrow 

 occipital ring; the front margin of the cephalon is broadly curved. Glabella very 

 faintly outlined on the interior of the cast; as thus shown it has a width at the 

 base of 6 mm. and at the front of 4.5 mm. on a cephalon n mm. in length; its 

 somewhat rounded front is about i mm. from the frontal rim of the cephalon; no 

 traces of glabellar furrows have been observed, and in only one specimen can the 

 very faint dorsal furrow that outlines the glabella be seen. 



Fixed cheeks of the same specimen 3 mm. in width at the palpebral lobes, from 

 which they extend with almost uniform width to the front, and broaden slightly back- 

 ward before merging into the short, triangular postero-lateral limbs; palpebral lobes 

 small and situated back of a line passing through longitudinal center of cephalon. 



The associated pygidium in the same fragment of rock is rounded subtriangular 

 in outline, moderately convex, and without any indication of an axis except a very 

 narrow, slightly marked median ridge on the cast of the interior. A specimen 1 1 mm. 

 in length has a width of 16 mm. at the front margin; a very slight elevation of the 

 front margin near the center indicates that the axial lobe of this specimen had a 

 width of about 6 mm. 



Surface minutely but not closely punctate under a strong lens. 



This species differs from /. caucus by the greater width of the cephalon at the 

 palpebral lobes and less convexity; the associated pygidium is less convex, more 

 subtriangular in outline, and without the indication of a central axis. 



The associated species on the same hand specimen are Ptychaspis ceto and 

 Coosia car me. 



Formation and Locality. Upper Cambrian: (038) Crystalline limestone near 

 the base of the Ch'au-mi-tien limestone [Blackwelder, 19070, p. 36 (part of first 

 list of fossils)], at Ch'au-mi-tien, Shan-tung, China. 



Collected by Eliot Blackwelder. 



