DESCRIPTION OF GENERA AND SPECIES. 79 



"The interior of the brachial valve, as far as known, shows no other characters 

 than the radiating lines, which appear to belong to the ornamentation of the outer 

 surface. 



" Shell substance tenuous, apparently corneous. External surface covered with 

 more or less prominent, sometimes lamellose concentric growth lines, crossed by fine, 

 gently curved, radiating striae which are usually more prominent when the concentric 

 lines are exfoliated." 



Discinopsis sulcatus (Walcott). 

 Plate 4, Figure 3. 



Craniella ?? sp. WALCOTT, 1905, Proc. U. S. Nat. Mus., vol. xxix, pp. 4 and 6. (Listed.) 

 Discinopsis sulcatus (WALCOTT), 1906, idem, vol. xxx, pp. 568-569. (Described anddiscussed as below 



as a new species.) 

 Discinopsis ? sulcatus (WALCOTT), 1912, Monogr. U.S. Geol. Survey, vol. LI, p. 721, plate Lxxxn,fig.6. 



(Copy of the preceding reference.) 



This species is based upon the cast of the interior of a small ventral valve, that 

 in its interior markings closely approaches the interior of the ventral valve of Dis- 

 cinopsis gulielmi (Matthew) [Walcott, 19126, plate LXXXII, figs. 5, $a-c\. 



The interior cast shows that the ventral valve was subcircular in outline, 

 moderately convex, and with the apex probably perforated by a small, circular 

 foraminal aperture. In front of the cast of the base of the foraminal aperture 

 there is a broad depression that extends to the front margin; on each side of the 

 central depression an elongate, slightly depressed area extends forward and outward 

 from near the base of the cast of the foraminal aperture, along the ridge on each 

 side of the median depression ; back of the base of the foraminal aperture there is 

 a narrow, short, arched furrow that indicates the presence of a corresponding ridge 

 on the interior of the shell. No other markings are shown on the cast, except the 

 faint outline of what may have been the visceral area, on the median line in front of 

 the base of the foraminal aperture and between the broad vascular sinuses. 



This species is referred to the genus Discinopsis as the result of comparison 

 with specimens of the interior of a ventral valve of D. gulielmi. One interior of 

 the latter species has scars much like those shown in D. sulcatus, figure 7. 



Formation and Locality. Upper Cambrian: (C5fi) Lower part of Ch'au-mi-tien 

 limestone, 25 feet (7.5 m.) below the top of Pagoda Hill [Blackwelder, 19070, p. 42 

 (part of last list of fossils)], i mile (1.6 km.) west of Tsi-nan, Shan-tung, China. 



Collected by Eliot Blackwelder. 



Genus BILLINGSELLA Hall and Clarke. 



For discussion of the genus Billingsella see Monogr. U. S. Geol. Survey, vol. 

 Li, 1912, pp. 749-75- 



Billingsella pumpellyi Walcott. 

 Plate 4, Figures 4, 4<z-c. 



Billingsella pumpellyi WALCOTT, 1905, Proc. U.S.Nat. Mus., vol. xxvin, p. 242. (Described and 



discussed as below as a new species.) 

 Billingsella pumpellyi WALCOTT, 1912, Monogr. U. S. Geol. Survey, vol. LI, pp. 760-761, plate xcvn, 



figs. 8, 8a-f. (Copy of the preceding reference.) 



General outline subsemicircular, greatest width at the hinge-line, or a little in 

 advance of it ; considerable variation exists in the relative proportions of length and 

 width; a ventral valve 8 mm. long has a width of 9 mm.; the dorsal valve is more 

 transverse, length 5.5 mm., width 8 mm. The ventral valve is strongly convex, 



