224 RESEARCH IN CHINA. 



Illaenurus dictys Walcott. 

 Plate 23, Figures 5. 5. 



Illirnurus dictys WAI.CUTT, 1905, Proc. U. S. Nat. Mus., vol. xxix, p. 98. (Described as a new species 

 essentially as below.) 



Cephalon, exclusive of the free cheeks, rounded, subquadrate, gently convex; 

 the posterior margin of the cephalon curves slightly upward opposite the central 

 portion, where there is a slight elevation which gives the appearance of a narrow 

 occipital ring; the front margin of the cephalon is broadly rounded, with a very 

 broad, obtuse angle at the center; the cast of the interior of the crust shows a very 

 faint, low, longitudinal median ridge. The glabella is not defined from the fixed 

 cheeks. The palpebral lobes are small and situated nearly opposite the center of 

 the cephalon; postero-lateral limbs small and short. 



Surface smooth under a strong lens. 



The associated pygidium is transverse, rounded, subtriangular; front broadly 

 rounded; sides gently rounded, forming a rounded obtuse angle at the posterior 

 margin; the cast of the interior of the crust shows a narrow, slightly defined axis, 

 with eight or more very faint transverse furrows and rings. The pleural lobes are 

 gently convex and without any trace of furrows. 



A cephalon of 6 mm. in length has an equal width at the palpebral lobes. A 

 specimen of the associated pygidium 7 mm. in length has a width of 8 mm. at the 

 frontal margin. 



This species differs from Illirnurus ceres [p. 223] in the obtusely pointed frontal 

 margin of the cephalon and its less convexity. From Illanurus canens [p. 222] it 

 differs in the direction of the facial sutures from the frontal margin back to the 

 palpebral lobes; the sutures of Ill&nurus dictys extend slightly outward from the 

 base of the palpebral lobe to the margin, while those of Illsnurus canens extend 

 directly forward, making the central portion of the cephalon narrower at the frontal 

 margin. 



Formation and Locality. Upper Cambrian: (('5(5) Lower part of Ch'au-mi-tien 

 limestone, 25 feet (7.5 m.) below the top of Pagoda Hill [Blackwelder, 190711, p. 42 

 (part of last list of fossils)], i mile (1.6 km.) west of Tsi-nan, Shan-tung, China. 



Collected by Eliot Blackwelder and Li San. 



SEPARATED PYGIDIA. 



There are several genera and a number of species represented in the collections 

 by pygidia which can not readily be referred to any known cephala. It is thought 

 best to indicate such by generic reference as far as possible, but not to give specific 

 names, thus avoiding duplication where the cephala of the same species may have 

 been described from other localities and by other authors. Where the separated 

 pygidia are associated with cephala considered to belong to the same genus, they 

 are placed next to the cephala on the plate, and notes made of their relations in the 

 description of the plate. 



There are also a number of free cheeks which are not associated with the 

 central portions of the cephalon of any species to which they can be assigned. A 

 number of these have been generically identified, and will be referred to in the 

 description of the plate upon which they arc figured. 



