268 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxviii. 



.sinus starts on each side of the base of the median ridge and arches 

 outward and then forward al)out a depressed oval space on each side of 

 tiie ridg-e; Uiteral l)ranches 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 depressions; 

 somewhat obscure radiating lines mark the anterior portion of the 

 surface. 



<)hs.i'riHitioni<. — 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 Plectorthls desiuop- 

 leura and Plectortlila vnchitaensts. 



Fwrnation <m(l locality. — Upper Cambrian. Chao Mi Tien lime- 

 stone, two-thirds miles west and Pagoda Hill, one mile west-southwest 

 of Tai An Fu, Shantung, China. 



Collection of Elliot Blackwelder, Carnegie Institution Expedition to 

 China. 



PLECTORTHIS PAPIAS, new species. 



This species is founded to receive a form represented by two small 

 dorsal valves from the Paradoxides zone of Newfoundland. They arc 

 moderately convex, with a low area divided by a broad delthyrium; 

 surface marked by numerous, rounded, depressed ribs, crossed by con- 

 centric strife and ridges of growth. Length of shell 4 mm. ; width, 

 5 imn. 



Formation and locality. — Middle Cambrian. Paradoxides zone. 

 Shales with Paradoxides davisi and 40 feet lower in the section. 

 Manuels river. Conception Bay, Newfoundland. 



PLECTORTHIS REMNICHA Winchell. 



Orthis remnlclm N. H. \VI^x^ELL, Fourteenth Ann. Keii. Geol. Nat. Hist. Sur- 

 vey Minnesota, 1886, \,. 317, pi. ii, fig. 7. 



Orthis f reiiinielia Walcott, Mong. XXXII, U. S. (leol. Survey, 1899, pp. 451, 

 452, pi. Lxi, figp. 3, .Sa; pi. lxii, figs. 1, la-d. 



Shell of medium size, usually slightly transverse, with an ol^long, 

 oval outline for the ventral valve, and a subquadrate to semicircular 

 outline for the dorsal valve. Valves moderately convex, with an 

 almost straight hinge line that varies in length from nearly the great- 

 est width of the shell to two-thirds the greatest width; cardinal angles 

 varying from !H>-' or less in the extreme forms, with extremities some- 

 what angular, to the other extreme, where they are very obtuse and 

 have the appearance of being almost rounded, their angle being not 

 less than 120^, Cardinal area narrow, but well developed on each 

 valve, and divided by a rather large delthj^rium. 



The ventral valve has in some specimens a shallow mesial depres- 

 sion, and in some examples it is flattened toward the cardinal angles; 

 beak small and curviiig down toward the hinge line, beyond which it 



