CAMBRIAN BRACHIOPODA—WALCOTT. 581 



Formation and locality. — Upper Cambrian, 2 miles southeast and 

 1 mile southwest of summit of Benders Pass, Silver Peak Range, 

 Nevada. F. B. Weeks, collector, 1900. 



ACROTRETA BAILEYI Matthew. 



Acrolreta hailei/i ^Matthew, Trans. Roy. Soe. Canada, III, lcS86, p. ?A\ j)l. v, 

 fig. 13.— Hall and Clarke, Pal. Ne\¥ York, VIIT, 1892, Pt. 1, p. 10i>, pi. iii, 

 figs. 32-34.— Matthew?, Trans. Roy. Soc. Canada, IX, 1892, p. 43. pi. xii, 

 fig. 7d. Referred to A. biseda. 



Acrotreta bmcta Matthew, Bull. Nat. Hist. 8of. New Brunswick, lY, 1902, Pt. 5, 

 p. 395, pi. XVI, figs. 1, la-d. 



General outline transversely oval, with the posterior margin more 

 or less straight for less than one-half the transverse diameter of the 

 shell. On the pedicle valve the posterior margin is rounded in at the 

 median furrow of the false area, and on the brachial valve it curves 

 gently from the cardinal slopes to the beak. 



The pedicle valve is strongly convex, with the apex about one- 

 fourth the length of the valve from the posterior margin. The 

 crushed condition of the shells does not permit of decision as to 

 ^vhether the umbo or the apex is the most elevated part of the valve. 

 False area defined by the incurving of the cardinal slopes so as to 

 form a somewhat flattened triangular space that is divided midwa}^ by 

 a rather strong vertical furrow. Foraminal aperture rather large and 

 opening obliquel}^ backward. The brachial valve is gently convex, 

 with a minute marginal beak. 



Surface of shell marked b}^ fine concentric stride and lines of growth, 

 and possibly by fine radiating strise. I fail to find any traces of the 

 radiating strife on specimens that have not been distorted by pressure, 

 and it looks as thcfugh all radiating striae and lines are the result of 

 lateral compression of the shell. The average length is about 3.5 mm. 

 and the width 4 umi. Distorted shells give a length of 4 mm. and a 

 width of 3 mm. 



The cast of the interior of the pedicle valve shows a strong apical 

 callosity, a fair-sized foraminal tube, large cardinal scars, and a small 

 visceral area in front of the apical cal losity . Dr. G. F. Matthew describes 

 and shows on his illustration two "minute muscular scars" close to the 

 ""umbonal tubercle" and on each side of the parallel strite.' 1 find on 

 one specimen what may have led Dr. Matthew to believe there were 

 such scars, but on many others quite as well preserved there were none 

 such, and they are not known in any other species of the genus. 

 Messrs. Hall and Clarke examined Dr. Matthew's t^^pe material, and 

 say that with the original material before them they were unable "to 

 distinguish anything more than the central callosity.""^ 



1 Trans. Roy. Soc. Canada, III, p. 36, pi. v, fig. 13c. 

 •^Pal. New York, VIII, Pt. 1, 1892, p. 102.- 



