574 CAMBEIAN BEACHIOPODA. 



lateral muscles, were attached. Another shell from the Bighorn Mountains shows something 

 of the same character (PI. LIII, fig. 11). 



The area of the ventral valve is short, and divided midway by a shallow pedicle furrow; 

 area of dorsal valve unknown. 



The exterior of the shell appears to be smooth and slightly polished, except for fine con- 

 centric strise and lines of growth and what appear to be scattered punctse. 



The average size of the adult shells is from 3 to 4 mm., the length and width usually being 

 nearly the same. 



The exact horizon of the type specimen is unknown. It occurs in a somewhat friable, 

 purplish sandstone, unlike any beds in the Middle Cambrian Deadwood formation that I met 

 with. The specimens from the limestones of the Bighorn Mountains, etc., appear to belong 

 to the Upper Cambrian. 



Formation and locality. — Ordovician: (302v) Sandstones 2 miles (3.2 km.) west of the main summit of Buffalo 

 Fork Peak [St. John, 1877, pjj. 468 and 469 and map opposite p. 324 (near station XLIX)], about 25 miles (40 km.) 

 east of Jackson Lake, Uinta County, Wyoming. 



TTpper Cambrian: (152a) West side of Dry Creek below Pass Creek; and (lS8a) limestone north of East Gallatin 

 River near Hillsdale; both in the Threeforks quadrangle (U. S. Geol. Survey), Gallatin County, Montana. 



(302u) Upper limestone of the Deadwood formation, north of Cheever's ranch, on the summit of the Bighorn 

 Mountains, Wyoming. 



Middle Cambrian: (170) Sandstone about 10 miles (16.1 km.) south-southeast of Bald Mountain, Bighorn Moun- 

 tains; (302m) middle of shale above lower sandstone on Billy Creek in the Bighorn Mountains; (302y) sandstone on 

 Johnson Creek, Bighorn Mountains; and (302b) limestones near Crowfoot P.idge, Gallatin quadrangle (U. S. Geol. 

 Survey), Yellowstone National Park; all in Wyoming. 



(302z) Limestone on Mill Creek, near Landusky, Little Rocky Mountains, Chouteau County, Montana. 



(355) Sandstones on Red Canyon Creek, southwest side of Black Hills; and (355d) sandstone in the Black Hills; 

 both in South Dakota. ^ 



(llj) Basal part of Bonneterre limestone, Mine Lamotte, Madison County; and (11m) drill cores of limestone at 

 horizons 10 and 20 feet (3 and 6 m.) above the Lamotte sandstone, St. Francois County; both in Missouri. 



DiCELLOMUS PARVUS Walcott. 



Plate LXXXIX, figures 11, lla-d. 



Dicellomus parvus Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, pp. 315-316. (Described and discussed as below as a 

 new species.) 



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 apparent^ calcareocorneous. 



Ventral 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 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 anterior lateral muscle scars is indicated at the end of the 

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



Ohservations. — This minute shell has the generic characters of Dicellomus politus (Hall), but 

 it differs specifically in its minute size and the positions of the muscle scars in the dorsal valve. 



Formation and locality. — Middle Cambrian : (C6) Thin slahhy limestone in the upper shale member of the Kiulung 

 group [Blachwelder, 1907a, pp. 37 and 41 {Sd list of fossils), and fig. 10 {bed IS), p. 38], 2.5 miles (4 Im.) southwest of 



