ACEOTRETIDiE. 681 



two-fifths of the diameter of the valve ; tlie false area is fairly well defined by the abrupt incurv- 

 ing of the cardinal slopes; its median furrow is distinctly outlined, and it slopes forward to the 

 apex, which is situated about two-fifths the distance from the posterior margin to the fi'ont 

 margin. Pedicle aperture minute and directed upward from the extreme apex of the valve. 

 Dorsal valve gently convex. Surface of shell marked by fine concentric strise and lines of 

 growth, and very fine irregular, undulating striae that can be seen only by the aid of a strong lens. 



A partial cast of a dorsal valve from the same faunal horizon about 30 miles distant shows 

 a well-defined median ridge and cardinal scars. 



Ohservations. — This species is most nearly related to Acrotreta primseva Walcott of the 

 upper Olenellus fauna, at Pioche, Nevada. It differs in having a less elevated ventral valve 

 and in the more advanced position of the apex and foraminal aperture. All the specimens 

 known are also smaller than the average of A. primxva, the largest being 3 mm. in width and 

 2.5 mm. in length. 



The specific name is derived from Clayton Valley, near the type locality. 



FoEMATioN AND LOCALITY. — Lower Cambrian: (41) Limestone in the Pioche formation [Walcott, 1908a, p. 11] 

 on a ridge 2.5 miles (4 km.) northwest of the town of Cherry Creek, "V^Tiite Pine County, Nevada. 



(175) Shales carrying Olenellus on the divide between Clayton and Fish LaJce valleys, about IS miles {^4-^ km.) south- 

 southwest of Silver Peak; (184) shales at the summit of the Silver Peak Range, 10 miles (16.1 km.) southwest of the town 

 of Silver Peak; and (11) shales of No. 3 of the Silver Peak group. Barrel Spring section [Walcott, 1908f, p. ISQo], 2.5 

 miles (4 km.) south of Barrel Spring and 0.5 mile (0.8 km.) east of the road, in the extreme southeastern corner of 

 the Silver Peak quadrangle; all in the Silver Peak quadrangle (U. S. Geol. Survey), Esmeralda County, Nevada. 



ACEOTRETA CONCENTRICA Walcott. 



Plate LXXVII, figures 1, la-b. 



Acrotreta concentrica Walcott, 1902, Proc. U. S. Nat. Mus., vol. 25, p. 583. (Described as below as a new species.) 



Shell small, outline subcircular to transversely broad oval; apex of ventral valve nearly 

 central; false area indicated by a slight flattening of the valve on the posterior side, and a 

 distinctly marked, but very narrow, median furrow. The elevation of the valve is about one- 

 half its diameter. The dorsal valve is slightly convex and its cast is marked by small cardinal 

 scars and a narrow median ridge that extends to the anterior third of the shell. A ventral 

 valve is 1.25 mm. in diameter, and one dorsal valve is 2 mm. in length. 



Ohservations. — This species occurs at about the horizon of Acrotreta Tcutorgai Walcott but 

 is distinguished from that species and all others by the central position of the apex of the 

 ventral valve. 



This form owes its specific name to the nearly central position of the apex of the ventral 

 valve. 



Formation and locality. — Middle Cambrian: (96x) Limestone 1 mile (1.6 km.) north of Adairsville, 15 miles 

 (24.2 km.) northwest of Cartersville, Cartersville quadrangle (U. S. Geol. Survey), Bartow County, Georgia. 



Acrotreta conula Walcott. 

 Plate LXXV, figures 2, 2a-b. 

 Acrotreta conula Waicott, 1902, Proc. U. S. Nat. Mus., vol. 25, p. 584. (Described as below as a new species.) 



Shell small. Ventral valve eccentrically cone-shaped, with a broadly oval transverse 

 aperture. The apex is situated at the posterior third of the distance between the front margin 

 and the posterior edge of the slightly indicated false area. A minute pedicle aperture occurs 

 at the summit of the apex. Dorsal valve convex, with the minute beak at the posterior margin. 

 The cast shows a well-defined median ridge with the central muscle scars well back on the 

 valve. 



Surface marked by fine concentric stride and lines of growth. 



This form owes its specific name to the fact that the ventral valve is markedly cone-shaped. 



Formation and locality. — Tipper Cambrian : (310a) Shales of Olenus truncatus zone, Oeland Island, Sweden. 



o The species mentioned occur at two siiglitiy different localities (11 and Iv), none of the species being common to both localities. 



