CAMURIAX BRACHIOPODA WALCOTT 93 



above the sandy shales of the Big Cottonwood Canyon Section, Utah. 

 The associated fragments of trilobites indicate the IMiddle Cambrian 

 fauna of the Wasatch Range Section. 



This species differs from L. girtyi (Walcott) [1902, p. 602] in 

 not having a false area in the ventral valve and also in the position 

 of the foraminal opening. The incurved apex is more like L. 

 nwdesta (see p. 90), but the form of the ventral valve and the posi- 

 tion of the foraminal aperture are quite different. 



Formation and Locality. — Middle Cambrian : Limestones about 

 200 feet (61 m.) above the Cambrian quartzitic beds, about .25 mile 

 (.40 km.) below the Maxfield Mine, Big Cottonwood Canyon, west 

 front of the Wasatch Mountains, southeast of Salt Lake City, Utah. 



Genus ACROTRETA Kutorga [1848, p. 275] 

 ACROTRETA BELLATULA, new species 



Plate 9, Figures 4, 4a-/; 



General outline subcircular to transversely broad oval, with the 

 posterior margin of the ventral valve nearly straight beneath the 

 false area. The ventral valve is moderately convex, with the apex a 

 little in front of the posterior margin. False area defined by the 

 incurving of the cardinal slopes so as to form a somewhat flattened 

 triangular space that is divided midway by a narrow and rather 

 shallow vertical furrow ; pedicle aperture of medium size and open- 

 ing slightly backward. Dorsal valve nearly as convex as the ventral, 

 except that it curves down in the posterior portion to a minute mar- 

 ginal beak. 



Surface of the shell marked by fine, concentric striae and lines of 

 growth that show marked variations in their sharpness on different 

 specimens, the older shells having a dark, dull surface giving the 

 effect of a thin film over the stria;; a few traces of radiating striae 

 are shown on some shells. 



The average length of the larger shells is about 1.75 mm.; width, 

 2 mm. 



The interior of the ventral valve shows a small apical callosity, 

 minute pedicle tube, and vascular sinuses originating a little back of 

 the apical callosity; a cast shows that there were medium-sized, ele- 

 vated cardinal muscle scars. An interior cast of the dorsal valve 

 indicates a median ridge that extended about four-fifths of the dis- 

 tance from the area to the front margin. It also indicates medium 

 sized, elevated cardinal muscle scars. A second specimen shows a 

 shorter median ridge, with small, elevated central muscle scars about 

 half way between the posterior and front margins of the shell. 



