OBOLID^. 453 



Croix sandstone," at Van Ness quarry, Gibraltar Bluff, Lodi, Columbia County; and (338b) St. Lawrence formation 

 [Sardeson, 1896, p. 95], at Osceola, Polk County; all in Wisconsin. 



(339a) St. Lawrence formation [Sardeson, 1896, p. 95], at Otisville, Washington County; and (97 and 339c) Reeds 

 Landing, foot of Lake Pepin, Wabasha County; both in Minnesota. 



Obolus (Westonia) balticus Walcott. 

 '^ Plate XLVIII, figures 7, 7a-b. 



LinguleUa? sp. No. 4, Wiman, 1902, Bull. Geol. Inst. Univ. Upsala, vol. 6, pt. 1, No. 11, p. 52, PI. II, figs. 37 and 



38. (Locality given. The sisecimens represented by figs. 37 and 38 are redrawn in this monograph, PI. XLVIII, 



figs. 7a and 7b, respectively.) 

 LinguleUa f sp. No. 6, Wiman, 1902, idem, p. 52, PL II, fig. 34. (Locality given. The specimen represented by fig. 



34 is redrawn in this monograph, PL XLVIII, fig. 7.) 

 Obolus (Westonia) haltica Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, p. 334. (Characterized aa below as a new 



species.) 



This species is characterized by its elongate form, with the sides of the dorsal valves regu- 

 larly rounded from the back to the rounded frontal margin. The dorsal valves are also more 

 regularly and strongly convex than in other species occurring in the Cambrian sandstones of 

 Sweden. The outer surface is marked by concentric and radiating striae, crossed obliquely by 

 very fine, irregiilar transverse strife. The latter are only to be seen with a strong magnifier 

 and favorable light. 



For reference to the geologic horizon of bowlders on Eggegrund Island, similar to the ones 

 containing this species, see description of Oholus ( Westonia) hottnicus, page 454. 



The species derives its name from its occurrence in the North Baltic region. 



Formation and locality. — Middle? Cambrian: (311h) Drift bowlder of bituminous sandstone, No. '26 [Wiinan, 

 1902, p. 57], on Biluddeyi, about 20 miles (32. 2 km.) east of Gefle; and (311o) drift bowlder of bituminous sandstone. No. 1 

 [Wiman, 1902, p. 57], on Skalstenarne Island, just west of Eggegrund Island, about 25 miles (40 km.) east-northeast 

 of Gefle; both in the Province of Gefleborg, Sweden. 



(311t) Drift bowlder of mottled calcareous sandstone [Wiman, 1902, p. 57], at Ofverby, parish of Jomala, Aland 

 Island, Finland, Russia. 



, Obolus (Westonia) blackwelderi Walcott. 



Plate XXXIX, figures 10, lOa-c. 



Obolus (Westonia) blackwelderi Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, p. 335. (Described and discussed as 

 below as a new species.) 



General form elongate, with the width about one-half the length; front margin broadly 

 rounded; sides slightly arched up to the lateral slopes, which are quite straight in the ventral 

 valve and curved toward the rounded posterior end of the dorsal valve ; the beak of the ventral 

 valve is pointed and marginal. The convexity of the valves is moderate, that of the dorsal 

 apparently being the greater. Surface rnarked by concentric striae and lines of growth which 

 are crossed by irregular, fine, imbricating, more or less transverse lines; the transverse lines 

 trend slightly backward toward the sides of the valves. A dorsal valve 8 nma. in length has a 

 width of 5 mm. 



Ohservations. — Nothing is known of the interior of the valves. When the shell is broken 

 from the limestone, the outer surface usually adheres to the matrix. The best exteriors are 

 those of shells on the surface of the layers. Of the known species of Westonia, 0. ( W.) njoimani 

 Walcott, and 0. (W.) 'balticus Walcott are most nearly related to 0. (W.) UlackweMeri. The 

 latter is a larger shell and also less narrow proportionally where the gently arching sides pass 

 into the posterolateral slopes. 



The specific name is given for Prof. Eliot Blackwelder, of the University of Wisconsin, who 

 collected the specimens. 



Formation and locality. — Middle Cambrian: (CI and C2) Lower shale member of the Kiulung group [Black- 

 welder, 1907a, pp. 37 and 40 (part of the 3d list of fossils), and fig. 10 (bed 4), p. 38], 2 miles (3.2 km.) south of 

 Yenchuang; and (C6) thin slabby limestone in the upper shale manber of the Kiulung group [Blachoelder, 1907a, pp. S7 

 and 41 (2d list of fossils), and fig. 10 (bed 12), p. 38\, 2.5 miles (4 hm.) southwest of Yenchuang; both in the Sintai 

 district. Shantung, China. 



