642 CAMBRIAN BRACHIOPODA. 



Lower Cambrian: (57e) About 115 feet (35 m.) below the Middle Cambrian, in limestone correlated with the top 

 of Ic of the Mount Whyte formation on Mount Bosworth [Walcott, 1908f, p. 213], just above the tunnel on the north 

 shoulder of Mount Stephen, about 3 miles (4.8 km.) east of Field, British Columbia; (35c) drift blocks supposed to 

 have come from the Mount Whyte formation [Walcott, 1908f, p. 214], foimd on the south slope of Mount Bosworth, a 

 short distance nortliwest of the Canadian Pacific Railway track between Stephen and Hector, eastern British Columbia; 

 and (35e) about 270 feet (82.3 m.) below the Middle Cambrian, in a greenish siliceous shale correlated with No. 3 of 

 the Mount Whyte formation on Mount Bosworth [Walcott, 1908f, p. 214], in the amphitheater between Popes Peak and 

 Mount Whyte, about 3 miles (4.8 km.) northwest of Lake Louise, southwest of Laggan on the Canadian Pacific Railway, 

 Alberta, Canada. 



(4v) About 200 feet (61 m.) above the unconformable base of the Cambrian and 75 feet (22.9 m.) above the top of 

 the cjuartzitic sandstones, in a shale which corresponds in stratigraphic position to shale No. 6 of the Dearborn River 

 section [Walcott, 190Sf, p. 202], on Gordon Creek, G miles (9.6 km.) from South Fork of Flathead River, Ovando quad- 

 rangle (U. S. Geol. Survey), Powell County, Montana. 



AcROTHELE CORIACEA Liiuiarssoii. 



Plate LVI, figures 1, la-i. 



Acrothele coriacea Linnaesson, 1876, Bihang till K. svensk Vet.-Akad. Handl., Bd. 3, No. 12. pp. 21-23, PL IV, figs. 



44-^8. (Described and discussed in English as a new species.) 

 Acrothele coriace Linnarsson, Broggee, 1878, Nyt Magazin for Naturvidenskabeme, Bd. 24, Hft. 1. p. 76, PL IV, 



fig. 11. (Mentioned.) 

 Acrothele coriacea Linnarsson, Geonwall, 1902, Danmarks geol. Undersffgelse, Rsekke 2, No. 13, p. 40. (Localities 



mentioned in Norwegian.) 



General form subcircular to transversely broad oval in outline, with the posterior margin 

 slightly transverse. Ventral valve slightly conical with the apex usuall}^ witliin the posterior 

 fourth of the distance from the front to the back margin. The apex, as usually seen, is foniied 

 of two minute, elongated tubercles with a narrow, elongate depression between them; at the 

 posterior end of tliis depression a minute pedicle aperture occurs; on some shells the two 

 minute tubercles unite in front so as to form a ring with a break at the back side just behind 

 which the pedicle aperture opens. The size of the tubercles and median depression varies in 

 different shells, and also with the amount of exfoliation of the outer shell. In some examples 

 when the shell is nearly all exfoliated at the apex and umbo the two tubercles are ])rominent 

 and a third tubercle occurs back of them which is the cast of the inner portion of the foraminal 

 tube. A narrow false area is faintly defined on some shells and more distinctly on others. A 

 median line often extends from the f oramuial aperture to the margin. Casts of the interior show 

 a much more distinctly defined false area, also that it is impressed in the shell. The striae of 

 growth cross the area without interruption except as they curve slightly upward at the center 

 on some shells. Linnarsson [1876, p. 21] states that there is no trace of a longitudinal groove 

 as in Acrotreta suhconica Kutorga; this is true of most specimens, but on others from the Para- 

 doxides forchhammeri zone, I find a narrow line and on others a curving upward of the concentric 

 striae at the median line. The dorsal valve is slightly convex near the umbo, from which it 

 curves rather abruptl}^ down to the posterior margin and in some shells the lateral and anterior 

 margins curve up a little, leaving a slightly concave surface between the margins and the umbo. 

 The apex is near the posterior margin; it is divided midway by a narrow furrow so as to form 

 a narrow elongate tubercle on each side. On some shells a slight depression or flattening 

 extends to about the middle of the valve. 



The exterior surface is marked by concentric lines and striae of growth and slight undula- 

 tions or ridges on the older shells ; a few rather faintly indicated, radiating costae occur on some 

 examples of the ventral valve. Linnarsson [1876, p. 21] describes the outer surface as being 

 rough, somewhat like leather. With a strong magnifier I find this to be caused by a minute 

 granulation on fine, irregular, slightly elevated striae. The shiny inner layers of the shell 

 are marked by concentric lines and very fine radiating lines. 



The shell is corneous and built up of numerous thin layers or lamellae so as to form a 

 sheU that is strong at the umbo and thin toward the margins. The largest ventral valve in 



