ACROTRETID^. 669 



section. It differs from L. nitens, wliich occurs a short distance above it in the section, and 

 with which it is also associated, in having a less elevated ventral valve and a less convex dorsal 

 valve; and from L. transversa in being more rounded in outline and less convex. It appears 

 to be more nearly related in form to L. minuta (Hall and Whitfield); it differs, however, 

 materially in the character of the interior markings of the ventral valve, features which also 

 separate it from L. girtyi Walcott and L. tennesseensis Walcott. 



Formation and locality. — Upper Cambrian: (30k) 1,150 feet {350.5 m.) above the Middle Cambrian and 2,175 

 feet (662.9 m.) below the top of the Upper Cambrian, at the top of the arenaceous shales and -Urruistones forming le of the 

 Orr formation [Walcott, 1908f, p. 176]; and (31t) about 1,175 feet (358.1 m.) above the Middle Cambriaa 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]; both on Orr Ridge, about 5 miles (8 km.) south of Marjum Pass, House Range [Walcott, 1908f, PI. XIII], 

 Millard County, Utah. 



(^ LlNNARSSONELL.4. NITENS Walcott. 



Plate LXXVIII, figures 7, 7a. 



Linnarssonella nitens Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, p. 91, PI. IX, fig. 7. (Discussed as 

 below aa a new species. Fig. 7 is copied in this monograph, PL LXXVIII, fig. 7.) 



The outline of the valves and the exterior appearance of this shell are much hke those of 

 L. modesta. It differs in having on the ventral valve a low, distinct area, with the perforated 

 apex directed backward on nearly the same plane as the umbo, or most elevated portion of 

 the valve. The dorsal valve is more convex and its interior has a strong median ridge extend- 

 ing forward from the posterior margin about two-thirds of the distance to the front margin, 

 a feature but httle developed in L. modesta. From L. minuta (Hall and Whitfield) it differs 

 in the Mgher apex of the ventral valve, and from L. transversa in the latter character and in 

 the more circular outline of the aperture of the valves. 



A single specimen of L. modesta was found associated Math this species. 



In size L. nitens varies from 1.5 to 2 mm. in diameter. 



Formation and locality. — Upper Cambrian: (Sit) 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, 190Sf, 

 p. 176], on Orr Ridge, about 5 miles (8 km.) south of Marjum Pass, House Range [Wakott, 190Sf, PI. XIIT], Millard 

 Count!/; and (32g) about 2,575 feet (784.9 m.) above the Cambrian quartzitic sandstones, in a blue limestone about 

 2, miles (3.2 km.) southeast of Muskrat Spring, on the northwest face of Grantsville Peak, Stansbury Range, Tooele 

 County; both in Utah. 



Linnarssonella tennesseensis Walcott. 



i/ Plate LXXIX, figures 3, 3a-k. 



Linnarssonella tennesseensis Walcott, 1902, Pror. U. S. Nat. Mus., vol. ^5, pp. 604-605. (Described and discussed 



as below as a new species.) 

 Meekina prima Walcott, 1905, idem, vol. 28, p. 313. (Name referred to, but it was not defined or used elsewhere. «) 



General form of ventral valve rounded subtriangular, moderately convex, with the beak 

 curving gently over a very low false area nearly to the posterior margin. Pedicle opening 

 minute and situated a little in front of the beak. 



Surface of shell marked b}^ fine concentric stri^ and hues of growth. The interior surface 

 shows a few fine, radiating strige. Shell rather thick and built up of several thin layers or 

 lamellae. 



The cast of the interior of the ventral valve shows small cardinal scars and a minute pedicle 

 tube directed backward, very nmch as in Oholella atlantica Walcott (PI. LV, figs. If-h). The 

 area is sliort, narrow, and diA'ided by a deep, narrow pedicle furrow; on each side of the latter 

 the area projects into the valve so as to form a shelf, with a thickening at the interior angle 

 next to the pedicle furrow; the central visceral area is confined mainly to the deep median 

 groove; an elevated area about the anterior end of the groove probably served as the point 

 of attachment of the central muscle; the main vascular sinuses are very strong and project 

 well toward the front part of- the vaU^e; the composite cardinal muscle scars, wliich served 



a A shell js referred to as Meeldm prima, but before the generic or specific description was published I found that the specimens were casts 

 of the interior of Lmnarssonella tennesseensis. 



