ACROTRETID^. 69'3 



the posterior margin; false area indefinite, except for a rather strong, shallow furrow that 

 extends from the apex to the margin. The elevation of the valve is about two-thirds its 

 diameter at its aperture. Dorsal valve slightly convex, apex marginal. Surface of shell 

 marked by fine, concentric striiB and lines of growth that on the dorsal valve tend to form 

 low ridges toward the outer margins. 



Observations. — This species is the representative of the American Acrotreta idahoensis 

 sulcata Walcott. It has the same type of false area, and the ventral valve is of average height. 



The specific name is given in recognition of Li San, Mr. WilHs's faithful Chinese interpreter. 



Formation and locality. — Middle Cambrian: (C22) Changhia limestone in upper oolitic portion [Blackwelder, 

 1907a, pp. 22 and 33 (part of last list of fossils)], at Changhia, Shantung, China. 



ACEOTRETA MAR.JXJMENSIS Walcott. 



U - 



Plate LXXVIII, figures 2, 2a-d. 



Acrotreta marjumensis Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, pp. 94-95, PI. IX, figs. 2 and 2a. (De- 

 scribed and discussed as below as a new species. Figs. 2 and 2a are copied in this monograph, PL LXXVIII, 

 figs. 2 and 2c, respectively.) 



The general form of this species is much like that of Acrotreta idahoensis Walcott. The 

 ventral valve differs in having a more strongly marked and broader false area and in the 

 greater curvature of the apex over the false area. It is also less elevated, or convex, in pro- 

 portion to the size of the shell. It may also be compared with A. nehoensis Walcott, from 

 which it differs in being less elevated and in not having a well-indicated false pedicle furrow. 

 The dorsal valve is moderately convex and differs little from the dorsal valve of A. idahoensis 

 and A. nehoensis. 



The interior of the dorsal valve shows a narrow median ridge that, at the center and 

 toward the front of the shell, rises as a sharp, rather high, and very narrow ridge. This ridge 

 starts posteriorly from a subtriangular, somewhat elevated area which has a longitudinal 

 furrow crossing it. The cardinal and central muscle scars are very clearly defined; in some 

 cases the central scars are slightly depressed, and in other shells elevated above the general 

 surface of the interior of the shell. 



The specific name is derived from Marjum Pass, near the tj'pe locality. 



Formation and locality. — Upper Cambrian: (33d) Thin-bedded blue limestone at the base of the first high 

 poitit southwest of the J. J. Thomas ranch, on the east side of the Fish Spring Range, Juab County, Utah. 



Middle Cambrian: (lin) About 3,000 feet (914-4 m.) above the Lower Cambrian and 1,400 feet (426.7 m.) below 

 the Upper Cambrian, in the upper part of the limestone forming la of the Marjum limestone [Walcott, 1908f, p. 779], in 

 the, long cliff 2 miles (3.2 hm.) southeast of Marjum Pass, House Range [Walcott, 190Sf, Pis. XIII and XV], Millard 

 County, Utah. 



Acrotreta microscopica (Shumard). 

 ^-^ Plate LX\ai, figures 1, la-h, lo, 2, 2a-d. 



Discina microscopica Shumard, 1861, Am. Jour. Sci., 2d ser., vol. 32, p. 221. (Described as a new species.) 



The external outline, form, and convexity of the two valves are so fully illustrated by the 

 figures that I will only call attention to the variation in the outline of the elevation of the 

 ventral valve. In Plate LXVTI, figure Id", the false area slopes backward instead of forward 

 as in other specimens. The lines of growth show that the growth of the anterior portion of the 

 shell was much more rapid than on the posterior face, thus causing the apex to tip back toward 

 the area, a feature not observed in the other specimens illustrated. The concentric surface 

 striae are sharp, but very fine. The only ulterior of a ventral valve found is represented by 

 figures lo, lo', and lo". 



The typical forms from Texas are represented by Plate LXVII, figures 1, la-h, and lo, and 

 what appears to be an identical form from Nevada bj^ figures 2, 2a-d. The latter form shows 

 about the same range of variation in outline and position of the apex of the ventral valve as the 

 typical forms from Texas. This species belongs to the group of forms with a wide false area. 

 It differs from A. idahoensis Walcott and A. curvata Walcott in the form of the ventral valve. 



