452 ' CAMBEIAN BRACHIOPODA. 



lAngulella aurora (Hall), Hall and Clarke, 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, PL II, figs. 12 and 



13. (No text reference.) 

 Lingula aurora Hall, Sardeson, 1896, Bull. Minnesota Acad. Nat. Sci., vol. 4, No. 1, pt. 1, p. 95. (New localities 



mentioned.) 

 Obolus (Westonia) aurora (Hall), Walcott, 1901, Proc. U. S. Nat. Mus., vol. 23, p. 691. (Merely changes generic 



reference.) 

 Lingulella aurora (Hall), Grabat; and Shimbr, 1907, North American Index Fossils, vol. 1, p. 193. (Described.) 



General form broadly ovate, with the dorsal valve almost subckcular, except for the gentle 

 slope of the cardinal margins toward the low obtuse beak. The beak of the ventral valve is 

 almost invariably broken off, as shown in Plate XLVI, figures la to Id. If the cardinal slopes 

 are restored, as in figure Ic, the form of the beak is rather obtuse. Original convexity unknown, 

 as aU of the large number of specimens in the collection are compressed and often flattened on 

 the surface of the shaly sandstone. It appears to have been moderate like that of Lingulella 

 amfla (Owen) and Oiolus (Westonia) stoneanus (Whitfield). The shell appears to have been 

 built up of a thin outer laj^er and thin lamellose inner layers. On some of the casts of the valves 

 the character of both the inner and outer surface markings is shown. 



The outer surface of the shell is marked by concentric striae and lines of growth and a 

 complex system of lamellose striae. The latter are almost directly transverse near the sides of 

 the shell and curve slightly backward over the central portion, where they have a sharp zigzag 

 character that is highly characteristic (PI. XLVI, figs. Ig and Ih). When the outer surface of 

 the shell is exfoliated the surface is marked by concentric lines and striae and fine radiating striae 

 (figs. 1 and lb). This latter surface is probably caused by the markings of the inner surface of 

 the shell and the inner surface of the outer layer or lamella, as is seen so frequently in the shell 

 of Lingulella acutangula (Roemer) (PL XVII) and Dicellomus folitus (Hall) (PI. LII). The 

 appearance of several specimens leads to the conclusion that the interior surface was more or 

 less punctate. It frequently happens that the outer surface of the shell is worn nearly smooth, 

 the only remaining traces of the beautiful surface ornamentation being toward the beak, or a 

 spot here and there protected by a strong raised line of growth. 



This is one of the largest sheUs of the genus. A ventral valve from Mazomanie is 21 mm. 

 long, with a maximum width of 17 mm., and an associated dorsal valve is 17.5 mm. long and 17 

 mm. in width. The cardinal area is well defined in both valves. In the ventral valve it is 

 divided at the center by a strong pedicle groove and toward its margins by narrow, clearly 

 defined flexure lines. The striae of growth cross the area parallel with its base (PI. XLVI, fig. Ic) . 



The area formed a thin shelf between the pedicle groove and the lateral margins, and the 

 undercut extended back beneath the area, but how far can not be determined, as the cast of it 

 is broken away in the specimens in the collection. ' 



The area of the dorsal valve is short as compared with that of the ventral valve. It extends 

 weU out on the cardinal slopes, and has a very faintly indicated flexure line near the lateral 

 margin. The cast of the visceral cavity (v) is partly shown in figure la. There is a peculiar 

 apron-like area that extends forward from the flattened cast of the pedicle groove (PI. XLVI, 

 figs. Ib-d) that probably represents a broad groove that existed in the interior of the shell 

 back of the visceral cavity. No clearly defined muscle scars have been observed. 



Ohservations. — The surface ornamentation is very characteristic, and is of the same general 

 type as that of Oholus (Westonia) ella. (Compare PI. XLVI, fig. Ih, with PL XL VII, fig. lo.) 



The outline of the valves is somewhat like that of Lingulella am/pla (Owen) (PL XXVIII). 

 It difl^ers in being wider in proportion to the length. As pointed out by HaU, Oholus (Westonia) 

 aurora is from the upper beds at Mazomanie and Lingulella ampla from the lower beds at 

 Trempealeau. 



PoEMATioN AND LOCAUTT. — ^Upper Cambrian: (S85x)o In the upper beds of the "St. Croix sandstone," near Mazo- 

 manie, Dane County; (85s) "St. Croix sandstone," at Prairie du Sac, Sauk County; (78, 78c, and 78s) "St. Croix sand- 

 stone" at Osceola, Polk County; (79) "St. Croix sandstone," in bluff near Hudson, St. Croix County; (86) "St. 



a S86x is the type locality, though the specimens in the United States National Museum collections to which that number is assigned were 

 collected much later than the type specimens. The species also occurs in Localities 3281 and 328p. ■ 



