572 PROCEEDIXGS OF THE XATIOXAL MUSEUM. vol. xxx-. 



\ated ridge, that extends around the border of the elevated portion of 

 the shell, except on what is supposed to be the posterior side; on this 

 side the curvature from the niaroin across the bordei' to the elevated 

 portion of the shell is uninterrupted. 



A second specimen occurs in the collection which shows the exterior 

 surface. This is smooth, and corresponds in surface configuration 

 approximatel}' to the reverse of the specimen above described. Unfor- 

 tunatel}' it is in a very fragmentar}^ state, neither the outline of the 

 margin nor the central, elevated portion being preserved. 



Ohservfdions. — This species is distinguished In' the broad, campanu- 

 late-like border; it ditfers from Scenella cJotlio Walcott" in its smooth, 

 exterior surface, and the strong, radiating strise on the inner surface, 

 both the outside and inside of the shell of S. clotho being marked by 

 concentric strife and lines of growth. 



Formation and locality. — Middle Cambrian, lower portion of Ki- 

 chou formation, in a brown, oolitic limestone; 4 miles south-south- 

 west of Tung-3'ii-chon, Shen-si, China. 



Collected by Eliot Blackwelder. 



Genus PLATYCERAS Conrad. 

 PLATYCERAS WILLISI, new species. 



Shell minute, consisting of two whorls somewhat irregularly incurved; 

 the apex and one side of the aperture lie in the same plane: the outer 

 whorl increases in size gradualh' through its first half, and then 

 expands more rapidly toward the aperture; a cross section of the 

 outer whorl shows the dorsal (outer) side to be gently convex, and the 

 inner side somewhat more strongly convex, a rounded dorsal angle 

 being formed where the two sides unite on the outer Q{\(fQ. 



The surface is marked by concentric lines of growth parallel to the 

 aperture. 



The greatest diameter of the largest shell is !.♦> mm, 



Obsc/'rafio/hs. — This species ditfers from Platyecra.'< chron us V^^Xcoii'' 

 in having a more slender, rounded, outer whorl, without trace of the 

 dorsal ridge characteristic of that species. It ditfers from P. clytla 

 Walcott'' in being coiled on the plane of the dorsal side instead of on 

 the plane of the median line. 



Format Ion aiul locaJlty. — Middle Cambrian, in greenish grav lime- 

 stone, interl)eded in argillaceous shales; i miles east of Fang-lan-chon, 

 Shan-si, China. 



Collected by Eliot Blackwelder. 



«Proc. U. S. Nat. .Mu>^., XXIX, 1905, p. 12. ''Idem, p. 14. 



