14 FIWCEEDINGS OF THE NATIONAL MUSEUM. vol. xxix. 



Genus PLATYCERAS Conrad. 

 PLATYCERAS CHRONUS, new species. 



Shell minute, consisting of two whorls somewhat irregularly in- 

 curved, the inner whorl being on the plane of the dorsal (outer) side; 

 the outer whorl expands rapidl}^ toward the aperture, increasing more 

 on the right ventral (inner) side; a cross section of the outer whorl 

 shows the outer side to be slightly convex and the inner side strongly 

 convex, a rather sharp dorsal angle Ijeing formed where the two unite 

 on the outer side. 



A narrow, sharp ridge occurs about midwa}^ of the outer side of the 

 whorl, that is seen only when the outer surface is very perfectly 

 preserved; in one example the ridge has a narrow depression on the 

 outer side which makes a rather prominent feature of the surface; 

 the strife of growth arcii backward to this I'idge, indicating a sharp 

 but small dorsal sinuosity in the pei'istomc; on casts of the interior 

 neither the ridge nor the arching backward of the strije is shown. 



The surface of finely preserved specimens is marked b}^ concentric 

 lines of growth, a sharp ridge, and one or two very fine, elevated lines 

 subparallel to the ridge. 



Greatest diameter of shell 1.5 mm. ; dorso-ventral diameter of whorl 

 at aperture 0.75 mm.; lateral diameter 0.5 mm. 



This species appears to be most nearly related to Platyceras j>ri- 

 msevimi Billings. It difiers in its stronger dorsal angle and more 

 rapidlj expanding outer whorl. 



Formation and locality. — Middle Cambrian, central portion of C'hang 

 Hsia formation; at Yen Chuang and 2.5 miles south of Yen Chuaug; 

 also in clifi's 1 mile east of Chang Hsia, in upper portion of Chang 

 Hsia formation; iShangtung, China. 



Collected l)y Eliot Blackwelder, of the Carnegie Institution of 

 Washington Expedition to China. 



PLATYCERAS CLYTIA, new species. 



Shell minute, consisting of about two whorls. Whorls regularly 

 incurved, the plane of the coiling being nearly perfect with the 

 exception of a very slight inclination to the left when looking down 

 upon it from above (dorsally). Outer whorl ver}^ minute at origin, 

 increasing slightly for the first half of its turn and then rapidly 

 toward the aperture, which is rounded ovate, being narrowest at the 

 dorsum; a rounded dorsal ridge is formed on the outer whorl by the 

 convex slope of the two sides meeting at the dorsum. 



Surface apparently smooth in the half dozen specimens in the 

 collection. 



Greatest diameter 2.75 mm.; dorso-ventral diameter of whorl near 

 aperture 1.6 mm.; greatest lateral diameter 1.25 mm. 



