CAMBRIAN BRACHIOPODA—WALCOTT. 583 



ACROTRETA CLAYTONI, new species. 



The material representing- this species does not give sufficient data 

 to prepare a detailed description. The only pedicle valve is depressed 

 by compression in the shale; it is transversely broad oval in outline, 

 with the posterior margin nearly straight for a distance of two-fifths 

 of the diameter of the valve; the false area fairly well defined by the 

 abrupt incurving of the cardinal slopes; its median furrow is distinctl}" 

 outlined, and it slopes forward to the apex, which is situated about 

 two-fifths the distance from the posterior margin to the front margin, 

 Foraminal aperture minute and directed upward from the extreme 

 apex of the valve. Brachial valve gently convex. Surface of shell 

 marked by fine concentric stria? and lines of growth, and very fine, 

 irregular, undulating stria^ that can be seen only by the aid of a strong 

 lens. ,^ 



A partial cast of a brachial valve from the same faunal horizon, 

 about 30 miles distant, shows a well-defined median ridge and cardinal 

 scars. 



This species is most nearl}^ related to A. prlinxva of the upper 

 OJenellus fauna, at Pioche, Nevada. It differs in having a less elevated 

 pedicle valve and the most advanced position of the apex and foraminal 

 aperture. All the five specimens known are also smaller than the 

 average of A. primseva, the largest being 3 mm. in wid'th and 2.5 mm. 

 in length. 



For inat Ion and locality . — Lower Cambrian shales, with Olendlus, on 

 the divide between Claj^ton and Fish Lake valleys, north of Red 

 Mountain, Silver Peak Range; also, 2 to 5 miles south of Barrel 

 Spring and one-half mile east of road as shown on the map of the Sil- 

 ver Peak quadrangle, Nevada. 



ACROTRETA CONCENTRICA, new species. 



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

 pedicle valve nearly concentric; false area indicated by a slight flatten- 

 ing 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 brachial 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 pedicle valve is 1.25 mm. 

 in diameter, and one brachial valve is 2 mm. in length. 



This species occurs at about the horizon of A. I'ntorgal, but is dis- 

 tinguished from that species and all others b}- the concentric position 

 of the apex of the pedicle valve. 



Foi'inatkm and Jocal'dy. — ^Middle Cambrian, shaly limestones on 

 roadside between Dalton and Lafayette, Georgia. 



