DESCRIPTION OF GENERA AND SPECIES. 99 



This little shell appears to be a true Cyrtoccras. Occurring, as it does, well 

 down toward the base of the Ch'au-mi-tien limestone, the fauna of which is of 

 Upper Cambrian age, makes it of great interest, as it is the oldest known represent- 

 ative of the genus; and unless Volborthella is considered to be a cephalod, it is the 

 oldest known representative of that class. 



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

 tien limestone, 25 feet (7.5 m.) below the top of Pagoda Hill [Blackwelder, 1907(7, 

 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. 



TRILOBITA. 



Genus AGNOSTUS Brongniart. 

 Agnostus chinensis Dames. 

 Plate 7, Figures 4-6, 6<i. 



Agnostus chinensis DAMES, 1883, China, Richthofen, vol. iv, p. 27, plate 2, figs. 18, 19. (Species 

 described and discussed.) 



This species is described in detail by Doctor Dames. He compares it with 

 Agnostus canadensis Billings [1860, p. 304], A. cyclopyge Tullberg [1880, p. 26], from 

 Sweden, A. communis and A. neon Hall and Whitfield [1877, pp. 228, 229], from 

 Nevada, and A. josepha Hall [1863, p. 178], from Wisconsin, pointing out that this 

 species from China belongs to the group Longifrontes of Tullberg [1880, p. 13]. 

 Twenty years later Doctor Monke, when studying a species of Agnostus from Shan- 

 tung, referred to the form of the pygidium of Doctor Dames's species, calling atten- 

 tion to its narrowing anteriorly, and the presence of two transverse furrows on 

 the anterior portion of the axial lobe. Doctor Monke recognized the close resem- 

 blance in the cephalic shield of the two forms, but on account of the differences 

 in the associated pygidia, described the species in his material as A. koerferi [1903, 

 p. in]. With a large number of fine specimens of the cephalon of the latter species 

 for comparison I find that the range of variation in form from the same locality 

 includes both A. chinensis and .4. koerferi, but I do not find any pygidium similar 

 to that illustrated by Doctor Dames and on that account think that the A. koerferi 

 should be retained, although in my first review of Doctor Monke's species I placed 

 it under A. chinensis [Walcott, 1906, p. 564]. Some specimens have indications 

 of two transverse furrows on the side of the axial lobe opposite the central tubercle, 

 but none of them show furrows crossing the axial lobe. 



The cephalon and pygidium represented by figures 6 and 60, copied from Dames, 

 have evidently been somewhat compressed and flattened in the shaly limestone. 

 The compression of the frontal space of the cephalon has increased the sharpness 

 of the longitudinal furrow, a feature that is occasionally seen on specimens in our 

 collection, and the median lobe of the pygidium has been broadened and a longi- 

 tudinal crack made both in the lobe and the transverse ring in front of it. The 

 specimens illustrated by figures 5, 50-0, are convex and preserve their natural form, 

 and are identical with fine specimens recently received from southeast of Mukden. 



In the J. P. Iddings collection from Manchuria there is a fine series of specimens 

 representing this species both in limestone and argillaceous shale. It appears to 

 have a vertical range of about 200 feet (60 m.) above the basal sandstone of the 

 section. Some of the compressed pygidia in the shale are similar in appearance 

 to the pygidium illustrated by Dames, and which is copied in this paper on plate 4, 

 figure 6a. 



