570 PROCEEDINGS OF THE XATIOXAL MUSEU.V. 



Forii\atl()n and locality. — Middle Cambrian, central portion of 

 Ki-chou formation, in a dark gray limestone; 1 miles south-southwest 

 of Tung-3'u-clion, Shen-si, Cliina. 



Collected by Eliot BlackAvelder. 



Genus ORTHIS Dalman. 



PLECTORTHIS Hall and Clarke, subgenus of ORTHIS. 

 ORTHIS (PLECTORTHIS) AGRESTE, new species. 



Shell transverse, sul)seniicircular; a ventral valve '.> iinii. 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 arini 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, 6 ribs 

 in a space of 3 mm. near the front margin; a few of the ribs bifur- 

 cate, but most of them extend from the umbo to the front margin; the 

 ribs are crossed by fine, concentric strise and strong lines of growth. 



Ohser cat ions. — This shell is characterized by its regular convexit}' 

 and the uniformity of the rounded, radiating ribs. 



Formation and locality. — Middle Cambrian, near base of the Ch'ang- 

 hia formation in dirt}" gray, fossiliferous limestone; 1 mile east-south- 

 east of Ch'ang-hia, Shan-tung, China. 



Collected })y Eliot Blackwelder. 



ORTHIS (PLECTORTHIS) KICHOUENSIS, new species. 



This species is represented hy a specimen of the dorsal valve. The 

 outline of the shell is transversel}' rounded subquadrilateral; length, 

 9 nun.; width, 13 mm.; the highest point above the plane of the mar- 

 gin is about 3 mm. above the hinge line; a shallow median sinus begins 

 at the nmbo, and gradually widens toward the front margin; back of 

 the umbo the shell extends beyond the hinge line, and then recedes to 

 the beak, which apparently is at or a little over the upper edge of a 

 very narrow area. 



Surface marked by numerous radiating, rounded ril)s, with narrow 

 interspaces, 7 ribs in a distance of 5 mm.; a few bifurcations of the 

 ribs occur at irregular intervals between the apex and the margins. 

 There are no traces of concentric strife; if on the shell originally, they 

 have been removed ])v the wearing oil' of the outer surface. 



Ohservation.s. —This species is distinguished from all other species 

 by the strong, incurved umbo, and rounded ribs with narrow inter- 

 spaces. 



Format ion und locality. — Middle Cambrian, in lower part of Ki-chou 

 formation, in brown-gray, oolitic limestone; 4.5 miles south of Wu-t'ai- 

 hien, Shan-si, China. 



Collected by Eliot Blackwelder, 



