DESCRIPTION OF GENERA AND SPECIES. 81 
A single specimen of a fragmentary ventral valve of a species of Billingsella 
that may belong to this species occurs in the central part of the Man-t’o shale. It 
was found (C20) [Blackwelder, 19074, p. 26 (last list of fossils), and fig. 6 (bed 14), 
p. 25] on the west side of an isolated butte 1 mile (1.6 km.) south of Ch’ang-hia, 
Shan-tung, China. 
Collected by Eliot Blackwelder. 
Genus EOORTHIS Walcott. 
For discussion of the genus Eoorthis see Monogr. U. S. Geol. Survey, vol. 11, 
1O12, poyyoe 
Eoorthis agreste (Walcott). 
Plate 4, Figures 6, 6a. 
Orthis (Plectorthis) agreste WaLcoTt, 1906, Proc. U.S. Nat. Mus., vol. xxx, p.570. (Described as 
below as a new species.) 
Eoorthis agreste (WALcorTt), 1912, Monogr. U.S. Geol. Survey, vol. LI, pp. 773-774, plate LXxx1x, figs. 
6, 6a. (Copy of the preceding reference.) 
Shell transverse, subsemicircular; a ventral valve 9 mm. in length has a width 
of 12 mm. and a hinge-line 10.5 mm. in length; ventral valve moderately and 
regularly convex, with the apex curved down to an area that is slightly inclined 
backward from the hinge-line; the details of the cardinal area are unknown. 
Surface marked by rounded, radiating ribs and interspaces, six ribs in a space 
of 3 mm. near the front margin; a few of the ribs bifurcate, but most of them extend 
from the umbo to the front margin; the ribs are crossed by fine, concentric strize 
and strong lines of growth. 
This shell is characterized by its regular convexity and the uniformity of the 
rounded, radiating ribs. 
Formation and Locality—Middle Cambrian: (C28) Thin-bedded oolitic lime- 
stone at the base of the Ch’ang-hia limestone [Blackwelder, 19074, p. 32 (first list 
of fossils), and fig. 6 (bed 20), p. 25], just above the shales in the face of the cliff 
1 mile (1.6 km.) east-southeast of Ch’ang-hia, Shan-tung, China. 
Eoorthis doris (Walcott). 
Plate 4, Figures 7, 7a. 
Orthis (Plectorthis) doris WALCOTT, 1905, Proc. U.S. Nat. Mus., vol. xxvill, pp. 262-263. (Described 
and discussed as below as a new species.) 
Eoorthis doris (WaLcotTr), 1912, Monogr. U. S. Geol. Survey, vol. 11, p. 779, plate xcvil, figs. 13, 134. 
(Copy of the preceding reference.) 
Of this species a ventral and a dorsal valve occur in the collection, The general 
form is rounded subquadrangular. ‘The ventral valve is strongly convex, with the 
greatest height at the umbo, from which the beak arches over a high, backward 
inclining area. The dorsal valve is much like the ventral in form, except that it 
is less convex and less elevated at the area. Surface of both valves with numerous 
radiating ribs that have an angular summit and an angular depression between 
them; the ribs increase in number toward the front by interpolation. The ventral 
valve has a length of 7 mm.; width,8mm. The dorsal valve is a little shorter than 
the ventral. 
This shell is not unlike Eoorthis indianola (Walcott) [1912b, plate xctv, figs. rt, 
2, 2a-h] in form and surface characters. It differs in being more convex and in 
the absence of a sinus or median fold on either valve. 
