76 SMITHSONIAN misce;llane;ous coIvIvEctions vol. 53 



ternal form is similar to that of Oboliis and the interior characters 

 might readily have been developed from that genus. 



Genus DICELLOMUS Hall [1873, p. 246] 

 DICELLOMUS PARVUS, new species 



Plate 8, Figures 2 and 2a 



General form ovate, with the ventral valve subacuminate and 

 dorsal valve broad oval to subcircular. \'alves 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. 



A'entral valve uniformly convex, except that the slopes toward the 

 cardinal margins are more abrupt than elsewhere ; apex appears to 

 be marginal. The interior of the valve shows a short, low median 

 ridge in the center of the visceral cavity ; on each side and a little 

 in front of the end of the median ridge are the trapezoidal areas 

 for the attachment of muscle scars ; rather small, composite cardinal 

 muscle scars occur close to the cardinal margins. 



Dorsal valve somewhat less convex than the ventral ; apex mar- 

 ginal. The interior of the valve shows well-defined composite car- 

 dinal 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 anterior lateral muscle scars is 

 indicated at the end of the median septum a little in advance of the 

 center of the valve. 



Observations. — This minute shell has the generic characters of 

 Dicellomus politus (Hall) [1861, p. 24], but it differs specifically in 

 its minute size and in the position of the muscle scars in the dorsal 

 valve. 



Formation and Locality. — [Middle Cambrian: (i) Ch'aiig-hia 

 limestone, 2.^ miles {4.02 km.) soHtlra'est of Yen-chuang, Sin-t'ai 

 District, Shaiititng, China; and (2) from a fine-grained bluish-black 

 limestone river drift block one mile (1.61 km.) south of Ch'ong-ping- 

 hien, on the Xan-kiang River, southern Shensi, China. 



