CAMBRIAN BRACHIOPODA — WALCOTT 95 



formation [JValcott, ipo8a, p. lo], south side of Marjurn Pass, in 

 cliff southeast of divide, House Range, Millard County, Utah. 



ACROTRETA OPHIRENSIS DESCENDENS, new variety 



Plate 9, Figures i and la 



In the Cambrian Section of the House Range, Utah, the typical 

 forms of Acrotreta ophirensis Walcott [1902, p. 591] occur in the 

 Marjurn formation; 1,550 feet (472.4 m.) higher in the section, in 

 the Weeks formation, there are great numbers of a shell identical 

 in many respects. This latter shell differs from the typical form in 

 being less convex in both the ventral and dorsal valves and in having 

 a more pointed, less curved apex on the ventral valve. 



Formation and Locality. — Middle Cambrian : About 3,750 feet 

 (1,143 "^O above the top of the Lower Cambrian and 650 feet 

 (198. 1 m.) below the Upper Cambrian, in shaly limestones of the 

 Weeks formation [Walcott, 1908a, p. 10], north side of Weeks 

 Canyon, 3.5 miles (5.63 km.) south of Marjum Pass, House Range, 

 Millard County, Utah. 



ACROTRETA RUDIS, new species 



Plate 9, Figure S 



Acrotreta kutorgai WAhCorr (in part) , 1902, Proc. U. S. National Museum, 

 XXV, pp. 589-590. (Described as a new species, but the description 

 included specimens belonging to both A. kutorgai and A. rudis.) 



The specimens illustrating this species are all more or less crushed 

 and flattened on the surface of the fine argillaceous shale in which 

 they are embedded in large numbers, and they are often in the con- 

 dition of casts of the interior of the valves. 



The ventral valve, as far as can now be determined, was elevated, 

 conical, and with the apex overhanging the false area, so that when 

 the shells were crushed down the posterior side disappeared beneath. 

 A few fragmentary specimens show the false area to have been dis- 

 tinctly defined, of medium height, and marked by a narrow, shallow 

 median furrow extending from the apex to the margin of the valve. 

 Dorsal valve subcircular, slightly transverse, gently convex, and with 

 a minute beak at the posterior margin. The pedicle opening appears 

 to have been of medium size and situated at the apex of the valve. 



Surface marked by lines of growth and very fine concentric striae 

 that continue across the false area and furrow. 



The cast of the interior of the ventral valve shows a small but 

 well defined visceral area in advance of the apex and a short, strong. 



