CAMBRIAN BRACHTOPODA—WALCOTT. 593 



ACROTRETA PRIMA^A, new species. 



An-otreta gemma Walcott, Bull. U. S. Geol. Sur., Xo. .30, 1886, pp. 98, 99, pi. 

 VIII, figs. 1, la, b.; Tenth Ann. Kept. V. S. Geol. Sur., 1891, p. 608, pi. 

 Lxvii, figs. 5c, 5d, 5e. 



Outline of valves transversely oval to subcircular. Pedicle ^alve 

 subconical, with the apex at the summit of the false area near to or 

 just above the posterior margin. The elevation varies from one-half 

 to two-thirds the diameter of the shell. The false area is quite clearly 

 defined in most specimens; it varies in width at the posterior margin 

 from one-third to nearl}^ one-half of the diameter of the shell; the 

 path of advance of the pseudodeltidium is marked usually b}' a nar- 

 row incised line similar to that of A. attemcafa. Foramina! aperture 

 minute and situated at the apex of the cone. Brachial valve slighth^ 

 convex, with a minute beak slightly incurved over the posterior mar- 

 gin. A broad, slighth' defined median sinus flattens the front and cen- 

 tral portion of the valve, but it is not perceptible toward the umbo 

 and beak. Surface of the shell marked b}^ fine concentric stri» and 

 occasional ridges of growth, and on some shells there is a very fine, 

 almost microscopic irregular concentric striation that gives a fretted 

 surface somewhat similar to that of Ohohm ( ^Ye>it<>n>(^) ella. The inner 

 lamellie of the shell and the interior surface have traces of fine radiat- 

 ing strife, that with the irregular concentric strije give a broken, sub- 

 imbricated, fretted efl'ect to the surface. The shell is formed of a thin 

 outer la\'er and several thin inner layers or lamelhe, the outer ones 

 of which are slightly oblique to the outer surface la3^er. The largest 

 shells have a transverse diameter of 5.5 mm., and a longitudinal diam- 

 eter of 5 mm. ; height of pedicle valve 3 mm. ; brachial valve 1 mm. 

 The interior of a slightly crushed pedicle valve shows the cast of 

 an apical callosity, strong vascular canals, and faint outlines of the 

 visceral cavity. The cast of an interior of a brachial valve has a long, 

 well-defined median ridge, cardinal scars, and faintly defined central 

 scars. 



(Jhs'rnxdions. — This, the oldest species of the genus known to me, 

 has all the essential characters of the type species, as far as the avail- 

 able information permits of comparison. It is one of the largest 

 shells of the genus, and is a striking feature of the fauna of the 

 higher portion of the Olenellus zone of Central Nevada. The nearest 

 s]iecies appears to be A. attenuata var. 



ForiiKithni and locality. — Lower Cambrian, upper portion of OJe- 

 ndliiH zone, associated with fragments of OlendhiH <j'dl>ertl in thin 

 layers of limestone interbedded in shales on east side of anticline just 

 above quartzite, Pioche, Nevada. 



Proc. N. M. vol. XXV— 02 38 



