688 CAMBRIAN BRACHIOPODA. 



shallow, vascular sinus. Something of this same character occurs in the dorsal valve of 

 Oholus (Linguloholus) spissus (Billings) (PL XVI, figs. 2, 2d). The cardinal scars are small 

 and situated close to the narrow area. Another cast of the interior (fig. le) shows the central 

 scars distinctly; also the presence of a thickening of the shell in the vicinity of the cardinal 

 scars. The median ridge is smaller and shorter than in Plate LXV, figure Ig. 



Observations. — The external characters of this species suggest Acrotreta curvata Walcott, 

 but the elevation and curvature of the ventral valve are not the same and the interior of the 

 dorsal valve is very distinct in its specific characters. There are no interiors of the form from 

 the Black Hills, but the external characters appear to be the same. These include the broad 

 false area, with its sUghtly marked groove, the position of the apex, and the curvature of the 

 false area. 



Acrotreta idaJioensis has been found in the Dunderberg shale of the Eureka district, Nevada, 

 and it extends up also to the base of the Pogonip limestone. The horizon is higher than that 

 of the type specimen, but the shells from the higher and lower horizons appear to be identical. 



Formation and locality. — Lower Ordovician : (201a) Pogonip limestone, east slope of the ridge east of Ham- 

 burg Ridge; and (202) Pogonip limestone on the summit of the ridge directly southeast of the Jackson mine, north- 

 west of Shadow Canyon; both in the Eureka district [Hague, 1S92, Atlas], Eureka County, Nevada. 



Upper Cambrian : (61) Limestone in the Dunderberg shale [Walcott, 1908f , p. 184], a little south of the Hamburg 

 mine, Eureka district [Hague, 1892, Atlas], Eureka County; and (313) limestone 0.75 mile (1.2 km.) east-northeast of 

 McGill post office, White Pine County; both in Nevada. 



(15d) Thin-bedded blue limestone near Cave Spring, on the east side of the Fish Spring Range, about 4 miles 

 (6.4 km.) south of the J. J. Thomas ranch, Juab County; and (301i) about 350 feet (106.7 m.) above the Middle Cam- 

 brian and 2,950 feet (899.2 m.) below the top of the Upper Cambrian, near the base of the arenaceous limestones form- 

 ing 2a of the Orr formation [Walcott, 1908f, p. 177], on Orr Ridge, about 5 miles (8 km.) south of Marjum Pass, House 

 Range [Walcott, 1908f, PI. XIII], Millard County; both in Utah. 



(5c and 54t) Limestones of the St. Charles formation [Walcott, 1908a, p. 6], about ^50 feet (76 m.) above the Middle 

 Cambrian; and (54u) limestones of the St. Charles formation about 100 feet (30.2 m.) above the Middle Cambrian; 

 both on the north side of Twomile Canyon near its mouth, 2 miles (3.2 km.) southeast of Malade, Oneida County, 

 Idaho. 



(56g) Limestone of the St. Charles formation [Walcott, 1908a, p. 6], in the valley of the stream which flows into 

 Mill Canyon from the west, about 6 miles (9.6 km.) west-southwest of Liberty and 15 miles (24.2 km.) west of Mont- 

 pelier. Bear Lake County, Idaho. 



Middle Cambrian: (88a) Limestone about 100 feet (30.5 m.) above the quartzitic sandstone at the base of the 

 Cambrian, in the northern suburbs of Deadwood; and (165) limestone on the east side of the valley, in raihoad cut 

 about 1 mile (1.6 km.) below the main part of Deadwood; both in the Black Hills, South Dakota. 



(302a) Limestones at several places on the south side of the Gallatin Valley, Gallatin County, Montana. 



(302b) Limestones near Crowfoot Ridge, Gallatin quadrangle (U. S. Geol. Survey), Yellowstone National Park, 

 Wyoming. 



(59g) Limestones immediately overlying the Spence shale member of the Ute limestone [Walcott, 1908a, p. 8], 

 in a saddle north of the creek which flows into Mill Canyon from the west, about 5 miles (8 km.) west-southwest of 

 Liberty and 15 miles (24.2 km.) west of Liberty, Bear Lake County, Idaho. 



(lie) About 2,750 feet (838.2 m.) above the Lower Cambrian and 1,650 feet (502.9 m.) below the Upper Cam- 

 brian, at the base of la of the Marjum limestone [Walcott, 1908f, p. 179], about 4 miles (6.4 km.) southeast of Antelope 

 Springs, in the spur at the junction of the Deseret and Swasey Spring roads [Walcott, 1908f, PI. XIII], House 

 Range, Millard County, Utah. 



Acrotreta cf. idahoensis. 



This species is represented by a single specimen of a somewhat imperfect ventral valve. 

 It is larger than the average shells of Acrotreta idahoensis Walcott, and appears to have had a 

 more incurved apex. The exterior layer is also duller and less distinctly striated than most 

 specimens of the latter species. The shell is built up of numerous thin layers or lamellae, more 

 or less oblique to the outer surface; the lamellae are marked by numerous fine radiating strise 

 and a few concentric striae. The general impression given by the shell is that it is an old shell 

 which has thickened by growth and become more robust and incurved than is usual in the 

 species with which it is compared. 



Formation and locality. — Middle Cambrian: (7j) Limestones at the north end of the Quinn Canyun Range, 1 

 mile (1.6 km.) northwest of the Italian Ranch foothills, Nye County, Nevada. 



