DESCRIPTION OF GENERA AND SPECIES. 7 1 



This species is represented by a few fragments, one of which shows that the 

 ventral valve is elongate and the apex acuminate. The shell was built up of several 

 layers or lamellae, as in characteristic forms of Obolus and its subgenera. The 

 interior surface of some of the lamellae is marked by fine radiating and concentric 

 striae; the outer surface, under a strong magnifier, shows fine, concentric, somewhat 

 irregular striae. 



Formation and Locality. Middle Cambrian : (C 72) Thin green-gray limestone 

 interbedded with ocherous and green clay shales, overlying the massive oolite in the 

 Ki-chou formation [Willis and Blackwelder, 1907, pp. 139 and 145 (third list of 

 fossils)], 4 miles (6.4 km.) east of Fang-lan-chon, Shan-si, China. 



Collected by Eliot Blackwelder. 



Genus DICELLOMUS Hall. 



For discussion of genus see Monogr. U. S. Geol. Survey, vol. LI, 1912, pp. 

 571-572. 



Dicellomus parvus Walcott. 



Plate 3, Figures 3, $a-d. 



Dicellomus parvus WALCOTT, 1905, Proc. U.S. Nat.Mus., vol. xxvm, p. 315. (Described and discussed 



as a new species essentially as below.) 

 Dicellomus parvus WALCOTT, 1912, Monogr. U. S. Geol. Survey, vol. LI, pp. 574-575, plate LXXXIX, 



figs, ii, ua-d. (An essential copy of the preceding reference.) 



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

 broad oval to subcircular; valves moderately convex. Surface of outer shell dark 

 and polished; it is marked, when not abraded, by fine, clearly defined, concentric 

 striae and occasional lines of growth. The largest ventral valve has a length of 2.5 

 mm., and a width of 2 mm. The shell is strong but not thick; shell substance 

 apparently calcareo-corneous. 



Ventral valve uniformly convex, except that the slopes toward the cardinal 

 margins are more abrupt than elsewhere; apex appears to be marginal. 



Dorsal valve somewhat less convex than the ventral; apex marginal. The 

 interior of the valve shows well-defined, composite cardinal muscle-scars, a narrow 

 median septum, and a faintly impressed main vascular sinus that curves outward 

 and forward at about one-third the distance from the outer margin to the median 

 septum; the central muscle-scars are small and situated back of the center of the 

 valve on each side of a low median swelling on which the median septum occurs; 

 the position of the antero-lateral muscle-scars is indicated at the end of the median 

 septum a little in advance of the center of the valve. 



This minute shell has the generic characters of Dicellomus pulitits ( Hall) [Walcott, 

 19126, pp. 575-578, plate LII], but it differs specifically in its minute size and the 

 position of the muscle-scars in the dorsal valve. 



Formation and Locality. Middle Cambrian: (CO) Thin, platy limestone in 

 the upper shale member of the Kiu-lung group [Blackwelder, 190712, pp. 37 and 41 

 (second list of fossils), and fig. 10 (bed 12), p. 38], 2.5 miles (4 km.) southwest of 

 Yen-chuang, Sin-t'ai district, Shan-tung, China. 



Collected by Eliot Blackwelder. 



Also from (032), a fine-grained, bluish-black limestone bowlder believed to 

 have come from the lower part of the Ki-sin-ling limestone [Blackwelder, 19071", 

 p. 272], collected in river drift i mile (1.6 km.) south of Chon-p'ing-hien, on the 

 Nan-kiang River, southern Shen-si, China. 



Collected by Bailey Willis and Eliot Blackwelder. 



