86 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 55 



ACROTHELE SPURRI, new species 



Plate 8, Figurb; 14 



Acrothele subsidua Walcott (in part) [not White], 1886, Bull. U. S. 

 Geol. Survey, No. 30, p. 109, pi. ix, fig. 4 (not figs. 40-c). (Locality- 

 mentioned in discussion of A. subsidua. The specimen represented by 

 fig. 4 is redrawn in this paper, pi. 8, fig. 14.) 



Acrothele subsidua Walcott (in part) [not White], 1891, Tenth Ann. 

 Kept. U. S. Geol. Survey, p. 608, pi. lxx, fig. i (not figs. la-c). 

 (Mentioned. Fig. i is copied from fig. 4 of preceding reference.) 



General form transversely ovate. Ventral valve depressed, with 

 an elevated apex a little in front of the posterior margin; from the 

 apex the surface slopes rapidly and then gradually to the front 

 margin and quite rapidly to the posterior margin ; the posterior 

 margin arches up from the plane of the sides of the valves about one- 

 half the height of the apex and gives to the valve the appearance of 

 being pushed up so as to throw the apex toward the front; a minute 

 pedicle opening truncates the apex at its posterior side ; a small, sub- 

 triangular, convex false area extends from just back of the apex to 

 the posterior margin. 



Surface of the epidermal layer marked by fine, concentric strias of 

 growth and a very minute granulation ; the inner laminations of the 

 shell are shiny and marked by numerous radiating striae and a few 

 concentric lines. Shell corneous and built up of numerous layers or 

 lamellae more or less oblique to the outer layer. 



Dorsal valve and interior of valves unknown. The largest ventral 

 valve has a length of 6 mm. ; width, 7 inm. 



Observations. — This very striking species is associated with 

 Acrotreta primcuva Walcott [1902, p. 593] and Olenellus gilherti 

 IMeek in the upper portion of the Olenellus zone of central Nevada. 

 Some fragmentary .specimens of this species were referred by me 

 [1886, p. 109] to Acrothele subsidua White [1874, p. 6], but with 

 better material and more thorough study the Lower Cambrian speci- 

 mens are referred to A. spurri. A spurri differs from A subsidua 

 in the more posterior position of its apex ; in its convex, distinctly 

 marked false area; in its strongly arched posterior margin, and in 

 its more finely granulated surface. The convex false area and 

 arched posterior margin are features also seen in A. woodzvorthi 

 (see p. 88) of the Lower Cambrian. 



Formation and Locality. — Lower Cambrian : Associated zvith 

 Olenellus in the shales and thin interbedded layers of limestone of 

 the Pioche formation [Walcott, 1908a, p. 11], just above the quartz- 

 ite on the east side of the anticline near Pioche, Lincoln County. 

 Nevada. 



