DESCRIPTION OF GENERA AND SPECIES. 8 1 



A single specimen of a fragmentary ventral valve of a species of Billingsclla 

 that may belong to this species occurs in the central part of the Man-t'o shale. It 

 was found (C20) [Blackwelder, 19070, p. 26 (last list of fossils), and fig. 6 (bed 14), 

 p. 25] on the west side of an isolated butte i 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. LI, 

 1912, p. 772. 



Eoorthis agreste (Walcott). 

 Plate 4, Figures 6, 6a. 



Ortlris (Plectortliis) agreste WALCOTT, 1906, Proc. U.S. Nat. Mus., vol. xxx, p. 570. (Described as 



below as a new species.) 

 Eoorthis agreste (WALCOTT), 1912, Monogr. U. S. Geol. Survey, vol. LI, pp. 773~774. plate LXXXIX, figs. 



6, da. (Copy of the preceding reference.) 



vShell 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. 



vSurface 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 striae 

 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, 19070, p. 32 (first list 

 of fossils), and fig. 6 (bed 20), p. 25], just above the shales in the face of the cliff 

 i mile (1.6 km.) east-southeast of Ch'ang-hia, Shan-tung, China. 



Eoorthis doris (Walcott). 

 Plate 4, Figures 7, 70. 



Ortliis (Plectortliis) doris WALCOTT, 1905, Proc. U.S.Nat. Mus., vol. xxvm, pp. 262-263. (Described 



and discussed as below as a new species.) 

 Eoorthis doris (WALCOTT), 1912, Monogr. U. S. Geol. Survey, vol. LI, p. 779, plate xcvn, figs. 13, 130. 



(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, 8 mm. The dorsal valve is a little shorter than 

 the ventral. 



This shell is not unlike Eoorthis indianola (Walcott) [19126, plate xciv, figs, it, 

 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. 



