84 RESEARCH IN CHINA. 



Shell transverse, subsemicircular ; a ventral valve n mm. in length has a width 

 of 15 mm., and a dorsal valve 8 mm. in length has a width of 13 mm.; hinge-line 

 a little shorter than the greatest width of the shell; cardinal angles vary from 75 

 to in ; valves moderately convex. Cardinal area narrow in both valves and 

 inclined backward from the hinge-line. Surface marked by equidistant, narrow, 

 low ribs, three in a space of 2 mm. near the front margin of a shell 10 mm. long, with 

 fine, radiating striae between them; the radiating ribs and striae are crossed by fine 

 concentric stria? and lines of growth. 



Ventral valve with a strong, somewhat angular median fold, rising from a 

 well-defined depression on each side of it, or it might be designated as a very strong 

 rib rising above the general surface of the valve from a broad median depression; 

 the lateral slopes are gently convex. Dorsal valve with a strong, angular median 

 depression, beginning at the posterior margin and gradually widening to the front; 

 the sides of the depression rise above the general surface of the valve and form 

 with the outer slope a well-defined, low ridge on each side that extends a little 

 forward on the front margin to fit into the depressions on each side of the median 

 fold of the ventral valve. 



The interior of a small dorsal valve has a broad, strong median ridge corre- 

 sponding to the depression on the exterior surface ; a main vascular sinus starts on 

 each side of the base of the median ridge and arches outward and then forward 

 about a depressed oval space on each side of the ridge; lateral branches extend 

 from the main sinus toward the sides ; the impressions of the anterior and posterior 

 adductor muscle scars occur on the slopes of the median ridge and the oval depres- 

 sions; somewhat obscure radiating lines mark the anterior portion of the surface. 



This shell is distinguished by its strongly marked median ridge on the ventral 

 valve and sinus on the dorsal valve. In general form and surface markings it is 

 not unlike Eoorthis dcsmopleura (Meek) [Walcott, 19126, plate xcvi, figs, i, ia-r] 

 and Eoorthis ivichitaensis (Walcott) [idem, plate xciv, figs, i, la-o, in]. 



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

 stone [Blackwelder, 19070, p. 42 (part of last list of fossils)], (C54) near top of lime- 

 stone knoll two-thirds of a mile (i.i km.) west of Tsi-nan, and (C56) 25 feet (7.5 m.) 

 below the top of Pagoda Hill, i mile (1.6 km.) west of Tsi-nan, Shan-tung, China. 



Collected by Eliot Blackwelder and Li San. 



Eoorthis sp. undt. (a) (Walcott). 



Plate 4, Figure 13 



Orthis (Plectorthis) sp. undt. WALCOTT, 1906, Proc. U. S. Nat. Mus., vol. xxx, p. 571. (Described and 



discussed essentially as below.) 

 Eoorthis sp. undt. c (WALCOTT), 1912, Monogr. U. S. Geol. Survey, vol. Li, p. 793, plate LXXXIX, fig. 7. 



(A copy of the preceding reference.) 



This species is represented by a single specimen of the ventral valve. Ventral 

 valve convex, with the apex curving gently downward from the highest point to the 

 cardinal area; transverse length, 2.5 mm.; width, 3 mm.; hinge-line a little shorter 

 than the greatest width;' cardinal area sloping slightly backward from the hinge-line. 



Surface marked by about sixteen strong, nearly regular, rounded, radiating 

 ribs, that are crossed by fine, concentric striae, lines of growth, and one strong ridge 

 indicating interruption of growth. 



This little shell was at first placed with Eoorthis agreste [p. Si], but further 

 study showed that its greater convexity, elevated apex, and stronger ribs distin- 

 guishjit from that species. 



Formation and Locality. Middle Cambrian: (C2(>) Near the top of the black 

 oolitic group in the uppermost layers of the Ch'ang-hia limestone [Blackwelder, 



