94 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



Observations. — This species belongs to the Acrotreta sagittalis 

 (Salter) [i866a, p. 285] group. The low convexity of the ventral 

 valve, posterior position of the apex, the strong cardinal muscle scars, 

 and the strong median ridge, cardinal and central muscle scars of 

 the dorsal valve are all features common to Acrotreta bellattda and 

 A. sagittalis. It differs from A. sagittalis in the more posterior posi- 

 tion of the apex and less convexity of the ventral valve, and from 

 A. deHnita Walcott [1902, p. 584] in its much smaller size and less 

 elevated ventral valve. 



Formation and Locality. — Middle Cambrian : 2,825 to 3,025 feet 

 (86 1. 1 to 922 m.) above the top of the Lower Cambrian and 1,400 to 

 1,600 feet (426.7 to 487.7 m.) below the Upper Cambrian, in the gray 

 thin-bedded limestone of the Marjum formation [Walcott, 1908a, 

 p. 10], south side of Marjum Pass, in cliff southeast of the divide, 

 House Range, Millard County, Utah. 



ACROTRETA MARJUMENSIS, new species 



Plate 9, Figures 2 and 2a 



The general form of this species is much like that of Acrotreta 

 idahoensis Walcott [1902, p. 587]. 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 ele- 

 vated, or convex, in proportion to the size of the shell. It may also 

 be compared with A. neboensis Walcott [1905a, p. 300], 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. neboensis. 



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 longi- 

 tudinal 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. 



Formation and Locality. — Upper Cambrian : Thin-bedded blue 

 limestone at the base of the high point southwest of the J. J. Thomas 

 ranch, on the east side of the Fish Spring Range, Juab County, Utah. 



Middle Cambrian: About 3,02^ feet {p22 m.) above the top of 

 the Lozver Cambrian and 1,400 feet {426."/ m.) below the Upper 

 Cambrian, in the gray, more or less thin-bedded limestone of Marjum 



