DESCRIPTION OP GENERA AND SPECIES. 89 



A narrow, sharp ridge occurs about midway 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 striae of growth arch backward to this ridge, indicating 

 a sharp but small dorsal sinuosity in the peristome ; on casts of the interior neither 

 the ridge nor the arching backward of the striae is shown. 



The surface of finely preserved specimens is marked by 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 aper- 

 ture, 0.75 mm.; lateral diameter, 0.5 mm. 



This species appears to be most nearly related to Platyceras primcsvum Billings 

 [1871, p. 220]. It differs in its stronger dorsal angle and more rapidly expanding 

 outer whorl. 



Formation and Locality. Middle Cambrian: (Cl) Lower shale member of the 

 Kiu-lung group [Blackwelder, 190711, pp. 37 and 40 (part of the third list of fossils), 

 and fig. 10 (bed 4), p. 38], 2 miles (3.2 km.) south of Yen-chuang, and (C4) lime- 

 stone nodules at the base of the lower shale member of the Kiu-lung group [idem 

 (second list of fossils), and fig. 10 (bed 4), p. 38], 3 miles (4.8 km.) southwest of 

 Yen-chuang, Sin-t'ai district, Shan-tung; also (018), dark gray oolitic limestone 

 about 400 feet (120 m.) above the base of the Ch'ang-hia limestone [idem, p. 33 (third 

 list of fossils)], in cliffs i mile (1.6 km.) east of Ch'ang-hia, Shan-tung, China. 



Collected by Eliot Blackwelder. 



Pelagiella clytia (Walcott). 

 Plate 5, Figure 14. 



Platyceras clytia WALCOTT, 1905, Proc. U. S. Nat. Mus., vol. xxix, p. 14. (Described and discussed 

 as a new species essentially as below.) 



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 very 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 

 1.5 mm.; greatest lateral diameter, 1.25 mm. 



This species differs from Pelagiella chronus [p. 88] and P. pagoda in the form 

 of the outer whorl, which expands more uniformly and has a broadly ovate section; 

 its form also distinguishes it from Platyceras primavum Billings [1871, p. 220]. 



Formation and Locality. Upper Cambrian: (047) Upper part of the Ch'au- 

 mi-tien limestone [Blackwelder, 190701, fig. 9 (top of bed i), p. 35], at the top of a 

 high col at Ch'au-mi-tien, Shan-tung, China. 



Collected by Eliot Blackwelder. 



Pelagiella pagoda (Walcott). 

 Plate 5, Figures 10, ioa. 



Platyceras pagoda WALCOTT, 1905, Proc. U. S. Nat. Mus., vol. xxix, p. 15. (Described and discussed as 

 a new species essentially as below.) 



Shell minute, consisting of about two whorls, of which the inner whorl is very 

 small and incurved so as to be seen best from the right dorsal or outer side. Whorls 



