652 CAMBRIAN BRACHIOPODA. 



The specific name is given in honor of Dr. C. H. Pander. 



Formation and locality. — Lower Cambrian: (4v) About ^00 feet {61 m.) above the unconformable base of the 

 Cambrian and 75 feet {t2.9 m.) above the top of the quartzitic sandstones in a shale which corresponds in stratigraphic 

 position to shale No. 6 of the Dearborn River section [Walcott, 1908f, p. Z02], Gordon Creeh, 6 miles {9.6 km.) from South 

 Forh of Flathead River, Ovando quadrangle { U. S. Geol. Survey); and (5j) the same stratigraphic horizon as 4v, about 

 6 miles (9.6 km.) west-northwest of Scapegoat Mountain, on the Continental Divide between Bar Creek and the 

 headwaters of the south fork of North Fork of Sun River, Coopers Lake quadrangle (U. S. Geol. Survey); both in 

 Powell County, Montana. 



ACEOTHBLE PRETIOSA (Blllings). ' 



Plate LVIII, figures 1, la-g. 



Obolella pretiosa Billings, 1862, Geol. Survey Canada, Paleozoic Fossils, vol. 1, pp. 68-69, figs. 61a-b. (Described 



and discussed as a new species.) 

 Obolella pretiosa Billings, 1863, Geol. Survey Canada, Fifteenth Kept. Progress, p. 230, figs. 239a-b. (No text refer- 

 ence. Figures are copied from preceding reference.) 

 Obolella pretiosa BilUngs, Chapman, 1863, Canadian Jour. Industry, Sci., and Art, new ser., vol. 8, p. 191, fig. 160b. 

 Obolella pretiosa Billings, Chapman, 1864, Minerals and Geology of Canada, p. 163, fig. 160b. (Copy of preceding 



reference.) 

 Obolella? pretiosa Billings, Walcott, 1886, Bull. U. S. Geol. Survey No. 30, p. 111. (Generic relations mentioned.) 

 Billingda pretiosa (Billings), Ford, 1886, Am. Jour. Sci., 3d ser., vol. 31, p. 467. (Merely changes generic reference.) 

 Elhania pretiosa (Billings), Ford, 1886, idem, vol. 32, p. 325. (Merely changes generic reference.) 

 Not Obolella {Linnarssonia) pretiosa Dawson, 1890, Trans. Roy. Soc. Canada for 1889, 1st ser., vol. 7, sec. 4, No. 3, 



pp. 53-54, figs. 26a-c. (Referred in this monograph to Acrotreta sagittalis.) 

 Not Linnarssonia pretiosa Hall and Clarke, 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, p. 70, PI. Ill, 



figs. 43-44. (Referred in this monograph to Acrotreta sagittalis.) 

 Linnarssonia pretiosa Schuchbrt (in part), 1897, Bull. U.S. Geol. Survey No. 87, p. 262. (Mere reference, but includes 



specimens referred to both Acrotreta sagittalis and Acrothele pretiosa.) 

 Acrothele pretiosa {Billings), Walcott, 1898, Proc. U. S. Nat. Mus., vol. 21, p. 402. (Merely cha,nges generic reference.) 

 Linnarssonia pretiosa (Billings), Walcott, 1901, Proc. U. S. Nat. Mus., vol. 23, p. 673. (Merely changes generic 



reference.) 

 ?Linnarssonia pretiosa Grabau and Shimer, 1907, North American Index Fossils, vol. 1, p. 200. (Described, but not 



figured, and it is impossible to tell whether the authors are discussing the Linnarssonia pretiosa that is now 



referred to Acrotreta sagittalis or the true Acrothele pretiosa.) 



General form transversely oval; convexity unknown, as all specimens are more or less 

 flattened in the shale. Ventral valve with the apex near the posterior margm; pedicle aperture 

 small, and at the apex, but whether on its posterior slope or summit can not be determined. 

 Dorsal valve with the beak marginal, and greatest elevation on the umbo on the posterior third 

 of the valve. 



Surface marked by numerous concentric lines and strise of growth and a varying number 

 of irregular, low, narrow, more or less interrupted, radiating ribs; in addition there is a minutely 

 roughened surface formed by very fine, irregular, elevated hues that have minute granules upon 

 them; tliis results in a surface much hke that of Acrothele matthewi (Hartt). 



The average diameter of the larger specimens is 5 mm., though there are many less than 

 2 mm. across. 



The cast of the interior of a ventral valve shows the cast of a small oval apical callosity; 

 the interior of the dorsal valve has a narrow median ridge that extends forward to the anterior 

 fifth of the length of the valve; the main vascular sinuses are narrow and have the usual direc- 

 tion from the median line near the posterior margin outward and then obhquely forward. The 

 only muscle scars observed are the cardinal scars on each side of the median ridge between the 

 main vascular sinuses and the posterior margin, and the elongate oval central scars in advance 

 of the transverse center of the valve. 



Observations. — Owing to the compressed and more or less distorted condition of the speci- 

 mens representing this species, it is difhcult to give an accurate description of it. The posterior 

 position of the apex of the ventral valve, concentrically striated surface, long median ridge in 

 dorsal valve, and position of central scars, give the species an assemblage of detailed characters 

 unknown in other species of the genus. 



Formation and locality. — Ordovician: (220a) Shales of the "Upper Sillery^^ {Lauzon of Logan), on Chaudiere 

 River at the Grand Trunk railroad bridge; and (319c [Billings, 1862b, p. 69]) shales at Cape Rouge, above Quebec; 

 both in the Province of Quebec, Canada. 



