84 RESEARCH IN CHINA. 
Shell transverse, subsemicircular; a ventral valve 11 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 111°; 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 aspace of 2mm. near the front margin of a shell 10 mm. long, with 
fine, radiating striae between them; the radiating ribs and striz are crossed by fine 
concentric strie 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 desmopleura (Meek) [Walcott, 1912), plate xcv1, figs. 1, 1a-7] 
and Eoorthis wichitaensis (Walcott) [idem, plate xciv, figs. 1, 1a—o, ru]. 
Formation and Locality—Upper Cambrian: Lower part of Ch’au-mi-tién lime- 
stone [Blackwelder, 19074, p. 42 (part of last list of fossils)], (C54) near top of lime- 
stone knoll two-thirds of a mile (1.1 km.) west of Tsi-nan, and (C56) 25 feet (7.5 m.) 
below the top of Pagoda Hill, 1 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. ¢ (WALCOTT), 1912, Monogr. U.S. Geol. Survey, vol. 11, 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 striz, lines of growth, and one strong ridge 
indicating interruption of growth. 
This little shell was at first placed with Eoorthis agreste [p. 81], but further 
study showed that its greater convexity, elevated apex, and stronger ribs distin- 
guish it from that species. 
Formation and Locality.—Middle Cambrian: (C26) Near the top of the black 
oolitic group in the uppermost layers of the Ch’ang-hia limestone [Blackwelder, 
