320 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxviii. 



collection average 3 nnn. in transverse diameter; the ventral is a little 

 longer than the dorsal valve. 



A partiall}" exfoliated ventral valve shows a well-marked visceral 

 area, extending forward about one-third of the length of the shell; 

 also narrow main vascular sinuses starting near the apex and extend- 

 ing obli(piely forward well into the valve, about midway between the 

 median line of the valve and the lateral margins. This neat little shell 

 is distinguislu^d by its nearly circular outline, low convexity, and 

 small size. 



Formation (iiid local tty. — Middle Caml)rian. Chang Hsia limestone, 

 Yen Chuang, Shantung, China. 



Collection of Eliot T. Blackwelder, Carnegie Institution Expedition 

 to China. 



OBOLUS NUNDINA, new species. 



This little shell difi'ers from (J. anceps^ to which it appears to be 

 most nearl}^ related, by its more nearly circular dorsal valve, and its 

 strongly concentrically-ridged surface. Its surface is formed by ele- 

 vated, rounded, concentric ridges that are somewhat irregular. Very 

 fine concentric striiB occur ])oth on the ridges and on the interspaces 

 between. 



FoniKition and locality. — Middle Cambrian. (>old Creek, Llano 

 County, Texas. 



OBOLUS OBSCURUS, new species. 



This species is represented by one interior of a small dorsal valve 

 wliich is very distinctive, the exterior of a crushed dorsal valve, and 

 numerous fragments of the shell scattered through the hard, dark 

 gray, shaly sandstone. The large dorsal valve has a length of 9 nnn., 

 width T nnn. The shell is strong and marked on its inner la^^ers by 

 radiating and concentric striae; the outer surface has numerous tine, 

 elevated, slightly irregular concentric stride that, with a strong lens, 

 give it a roughened appearance. 



An interior of a dorsal valve shows a well-developed area, the 

 greater portion of which is taken b}" the broad pedicle groove; a deep 

 umbonal cavity with a narrow median septum and obscure main vas- 

 cular sinuses on the outer margins of the oavit3^ Outside the sinuses 

 on the postero-lateral slope, relatively large muscle scars are indicated. 



With the data available no satisfactory comparisons can be made 

 with other species. O. ohscurus is a large strong shell of the general 

 type of Oholns rhca of the Middle Cambrian of Wisconsin. 



Formation, and locality. — Middle CJambrian. Ku San shale. Three 

 and one-half miles southwest of Yen Chuang, Shantung, China. 



Collection of Eliot T. Blackwelder, Carnegie Institution Expedition. 



