670 CAMBEIAN BRACHIOPODA. 



as the point of attachment of tlie transniedian and anterior hxterals, are situated close to the 

 posterior lateral margin of the valve. 



The dorsal valve has a very short and narrow area, with a small but distinct slightly con- 

 cave shelf -like projection in front of it; on each side near the posterior lateral margins a strongly 

 defined cardinal posterior muscle scar occurs that served as the point of attachment of the 

 transmedian, outside lateral, and middle lateral muscles. The central scars are a little in 

 advance of the center of the valve. None of the specimens show the main vascular canals 

 except at their base. The shells average about 3 mm. in diameter. 



Observations. — The material representing this species occurs as casts of the interior and 

 exterior surfaces, in a fiire argillaceous shale. It is owing to the removal of the shell substance 

 by solution that the cast of the area and area-like shelf of the dorsal valve is so clearly shown, 

 also the posterior muscle scars, visceral area, etc., of the ventral valve. 



The shell has the general outhne of Linnarssonella girtyi Walcott; but it differs in having 

 the ventral valve more acuminate, less convex, and in the apex curving over nearly to the 

 posterior margin. It has the outline of L. minuta (Hall and Whitfield), but not the strong 

 vascular sinuses and small cardinal scars of the ventral valve of that species. 



The perforate ventral valve and large cardinal muscle scars relate this species to Linnars- 

 sonella. The composite cardinal muscle scars, area, and projection of the inner angles of the 

 area into the area of the ventral valve, and also the interior of the dorsal valve, suggest Dicel- 

 lomus. The spondylium-hke shelf of the dorsal valve, indicated in the cast, may be only the 

 cast of the area projecting out into the valve (PI. LXXIX, fig. Ir, of Linnarssonella girtyi). 

 The cardinal scars and shelf-like areas of the valve separate this species from Oholella. 



Formation and locality. — Tipper Cambrian: (31t) About 1,175 feet (358.1 m.) above the Middle Cambrian and 

 2,150 feet (655.3 m.) below the top of the Upper Cambrian, in the limestone forming Id of the Orr formation [Walcott, 

 1908f, p. 176], on Orr Ridge, about 5 miles (8 km.) south of Marjum Pass, House Range [Walcott, 1908f, PI. XIII], 

 Millard County, Utah. 



Middle Cambrian: (13) Sandstones of the Rome formation, 1.5 miles (3.4 hm.) east of Post Oah Springs [Hayes, 

 1894, areal geology sheet], Roane County; (107b) shales in railroad cut in Bull Run, northwest of Copper Ridge [Keith, 

 1896b, areal geology sheet], 11 miles (17.6 km.) northwest of Knoxville, Knox County; and (12) sandy shale on Webbs 

 Ridge in gap west of Simpson's farm, 6 miles (9.6 km.) northeast of Knoxville, Knox County; all in Tennessee. 



Linnarssonella transversa Walcott. 



Plate LXXVIII, figures 6, 6a-c. 



Linnarssonella transversa Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, p. 92, PI. IX, fig. 6. (Discussed as 

 below as a new species. Fig. 6 is copied in this monograph, PI. LXXVIII, fig. 6.) 



This is a small shell tliat at first sight suggests L. modesta; it differs from that species in 

 the valves being more convex, transverse instead of circular, and in having stronger concentric 

 stria, and, in some instances, radiating strise. A few fragments of the interior of the dorsal 

 valve indicate that the main vascular sinuses are well out toward the outer margin, and that 

 small circular depressions occur on each side of the median hne, very much as in the dorsal 

 valve of L. girtyi Walcott (PI. LXXIX, fig. Ir), and that two small central muscle scars are 

 midway between the posterior and anterior margins of the valve. The average shell has a 

 width of 2 mm., with a length of from 1.5 to 1.75 mm. 



L. transversa differs by its transverse outUne from all other known species of the genus. 

 Formation and locality. — Upper Cambrian: (30j) About 950 feet (289.6 m.) above the Middle Cambrian and 

 2,450 feet (746.8 m.) below the top of the Upper Cambrian, near the base of the arenaceous shales and limestone forming 

 le of the Orr formation [Walcott, 1908f, p. 176], on Orr Ridge, about 5 miles (8 km.) south of Marjum Pass, House 

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



Linnarssonella urania Walcott. 



Plate LXXVIII, figures 9, 9a-c. 



Linnarssonella urania Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, pp. 92-93, PI. IX, figs. 9 and 9a. (De- 

 scribed and discussed as below as a new species. Figs. 9 and 9a are copied in this monograph, PI. LXXVIII, 

 figs. 9a and 9c, respectively.) 

 Shell minute, general form elongate oval. Ventral valve convex; apex minute and curved 



down nearly to the plane of the margin of the valve ; false area, if present, must be very narrow. 



