OBOLID^. 415 



, Obolus shansiensis Walcott. 



ly 



Plate XI, figures 7, 7a-c. 



Obolus shensiensis Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, p. 327. (Described and discussed as below as a new 

 species, the spelling shensiensis being in error because the type specimens came from a locality in the Province 

 of Shansi, not Shensi, although the species occurs in that province.) 



General form ovate, with the ventral valve broadly subacuminate and dorsal valve obtusely- 

 rounded. Valves moderately convex in the specimens embedded in a fine-grained, dark lime- 

 stone. Surface marked by fine, sharp, concentric striae and traces of irregular, obscurely 

 defined, low, radiating ridges; the interior layers show fine radiating striae and concentric lines; 

 the shell is strong and built up of numerous lamellae oblique to the outer surface. The largest 

 ventral valve has a length of 9 mm., width 7 mm. Nothing is known of the interior of the 

 valves. 



The form, surface markings, and shell structure are much like those of Oholus matinalis 

 (Hall) (PI. VIII) and 0. ^etonensis Walcott (PI. IX). In outline 0. shansiensis is more elongate 

 than 0. matinalis and less so than 0. tetonensis. 



This form owes its specific name to the occurrence of the type specimen in the Province of 

 Shansi, Chiaa. 



Formation and locality. — Middle Cambrian: (C37) Upper part of the Kichou limestone in dense black lime- 

 stone nodules in green-gray shales 10 feet beloio the base of the cliff limestone [Willis and Blachwelder, 1907, p. 146], 8 miles 

 {12.S hm.) south of Tinghianghien, Shansi; (C32) A fine-grained bluish-black limestone bowlder believed to have come 

 from the lower part of the Kisinling limestone [Blackwelder, 1907b, p. 272], collected in river drift 1 mile (1.6 km.) 

 south of Chonpinghien, on Nankiang River, southern Shensi; and (C71) i^ massive cliff-forming limestone in the central 

 portion of the Kichou formation [Willis and Blackwelder, 1907, pp. 139 and 145 (2d list of fossils)], 4 miles (6.4 km.) 

 southwest of Tungyu, Shansi; all in China. 



/ Obolus sinoe (Walcott). 

 Plate XXVI, figures 2, 2a-h. 



Obolus (Lingulella) sinoe Walcott, 1898, Proc. U. S. Nat. Mus., vol. 21, pp. 416^17. (Described and discussed as 

 below as a new species.) 



General form broad ovate, with the ventral valve broadly subacuminate and the dorsal 

 valve broadly ovate. Valves moderately convex, as far as can be determined from the series 

 of shells preserved in the fine-grained sandstone. A ventral valve 5 mm. in length has a width 

 of 4.25 mm. A dorsal valve 4 mm. in length has an equal width. 



The traces remaining of the exterior shell show it to have been marked by concentric 

 fines and striae of growth; when the outer layer is exfohated radiating striae cross the lamellae; 

 a fragment of the inner surface of the shell indicates that there were scattered pits or punctae 

 and fine radiating striae; the shell was relatively thick and formed of a thin outer layer and 

 several inner layers or lamellae, the lameUae of the anterior portion of the shell being arranged in 

 layers sfightly obhque to the outer surface of the shell. 



Casts of the interior of the ventral valve show a well-defined area marked by strong flexure 

 lines that occur midway between the lateral margins and the narrow, well-defined pedicle groove ; 

 striae of growth cross the area parallel with its base. The area of the dorsal valve is relatively 

 short and does not extend very far out on the cardinal slopes. The interior markings of the 

 ventral and dorsal valves show imperfectly the main vascular sinuses and visceral area, the 

 central and anterior lateral muscle scars, and a narrow median ridge. 



Ohservations. — This species occurs at the same relative stratigraphic horizon as Lingulella 

 ino (Walcott) (PI. XXVI) and is about the same size. It differs, however, in its more circular 

 form, which is persistent in a large number of shells. In form it more nearly resembles Obolus 

 rotundatus (Walcott) (PL XX) of the Upper Cambrian. It differs from that in being uniformly 



» This species is somewhat doubtfully identified Irom this locality. 



